Csharp 简明教程

C

C

C# 是一种由 Microsoft 开发并由欧洲计算机制造商协会 (ECMA) 和国际标准化组织 (ISO) 批准的现代、通用、面向对象编程语言。

C# 由 Anders Hejlsberg 和他的团队在开发 .Net Framework 期间开发。

C# 是为通用语言基础结构 (CLI) 设计的,CLI 包含可执行代码和允许在不同计算机平台和架构上使用各种高级编程语言的运行时环境。

以下原因使 C# 成为广泛使用的专业语言 −

  1. 它是一种现代、通用的编程语言

  2. It is object oriented.

  3. It is component oriented.

  4. 它易于学习。

  5. 它是一种结构化语言。

  6. It produces efficient programs.

  7. 它可以在各种计算机平台上编译。

  8. 它是 .Net Framework 的一部分。

Strong Programming Features of C

尽管 C# 构造紧密遵循传统的 C 和 C++ 等高级语言,作为面向对象编程语言,它与 Java 有很强的相似性,它有许多强大的编程功能,使其受到全球众多程序员的喜爱。

以下是 C# 的一些重要功能 −

  1. Boolean Conditions

  2. Automatic Garbage Collection

  3. Standard Library

  4. Assembly Versioning

  5. Properties and Events

  6. Delegates and Events Management

  7. Easy-to-use Generics

  8. Indexers

  9. Conditional Compilation

  10. Simple Multithreading

  11. LINQ and Lambda Expressions

  12. Integration with Windows

C

在本章中,我们将讨论创建 C# 编程所需工具。我们已经提到 C# 是 .Net 框架的一部分,并用于编写 .Net 应用程序。因此,在讨论运行 C# 程序的可用工具之前,我们了解一下 C# 与 .Net 框架有何关系。

The .Net Framework

Net 框架是一个颠覆性平台,可帮助你编写以下类型的应用程序:
  1. Windows applications

  2. Web applications

  3. Web services

Net 框架包括一个庞大的代码库,由 C# 等客户端语言使用。以下是 .Net 框架中的一些组件:
  1. Common Language Runtime (CLR)

  2. .Net Framework 类库

  3. Common Language Specification

  4. Common Type System

  5. Metadata and Assemblies

  6. Windows Forms

  7. ASP.Net and ASP.Net AJAX

  8. ADO.Net

  9. Windows Workflow Foundation (WF)

  10. Windows Presentation Foundation

  11. Windows Communication Foundation (WCF)

  12. LINQ

有关这些组件执行的作业,请参阅 ASP.Net - Introduction ,有关每个组件的详细信息,请查阅 Microsoft 的文档。

Integrated Development Environment (IDE) for C

Microsoft 提供了以下用于 C# 编程的开发工具:

  1. Visual Studio 2010 (VS)

  2. Visual C# 2010 Express (VCE)

  3. Visual Web Developer

后两者可以从 Microsoft 官方网站免费获取。使用这些工具,你可以编写各种 C# 程序,从简单的命令行应用程序到更复杂的应用程序。你还可以使用基本文本编辑器(如记事本)编写 C# 源代码文件,并使用命令行编译器编译该代码,并转换为程序集的,它也是 .NET Framework 的一部分。

Visual C# Express 版和 Visual Web Developer Express 版是 Visual Studio 的精简版,并且具备相同的外观。它们保留了 Visual Studio 的大部分功能。在本教程中,我们使用了 Visual C# 2010 Express。

你可以从 Microsoft Visual Studio 下载它。它会自动安装到你的电脑上。

注意:安装 express 版本需要有效互联网连接。

Writing C

尽管 .NET Framework 在 Windows 操作系统上运行,但有一些适用于其他操作系统的替代版本。 Mono 是 .NET Framework 的开源版本,其中包含一个 C# 编译器,它可以在包括各种 Linux 和 Mac OS 等几种操作系统上运行。请查看 Go Mono

Mono 的既定目的是不仅能够跨平台运行 Microsoft .NET 应用程序,还为 Linux 开发人员提供更好的开发工具。Mono 可以在包括 Android、BSD、iOS、Linux、OS X、Windows、Solaris 和 UNIX 在内的许多操作系统上运行。

C

在我们学习 C# 编程语言的基础构建模块之前,让我们看一下最低限度的 C# 程序结构,以便我们可以在后续的章节中将其作为参考。

Creating Hello World Program

C# 程序包含以下部分 −

  1. Namespace declaration

  2. A class

  3. Class methods

  4. Class attributes

  5. A Main method

  6. Statements and Expressions

  7. Comments

让我们看一下输出“Hello World”的简单代码 −

using System;

namespace HelloWorldApplication {

   class HelloWorld {

      static void Main(string[] args) {
         /* my first program in C# */
         Console.WriteLine("Hello World");
         Console.ReadKey();
      }
   }
}

编译并执行此代码后,会产生以下结果 −

Hello World

让我们看一下给定程序的不同部分 −

  1. 程序的第一行 using System; - using 关键字用于包含 System * namespace in the program. A program generally has multiple *using 语句。

  2. 下一行是 namespace 声明。 namespace 是一个类集合。HelloWorldApplication 命名空间包含类 HelloWorld。

  3. 下一行是 class 声明,类 HelloWorld 包含您的程序使用的 data 和 method 定义。类通常包含多个 method。不过,HelloWorld 类仅有一个 * Main * method。

  4. 下一行定义 Main method,这是所有 C# 程序的 entry point 。*Main * method 指示在执行时类将执行的操作。

  5. 下一行 / …​ / 被编译器忽略,它用于在程序中添加 * 注释*。

  6. Main method 通过语句 Console.WriteLine("Hello World"); 指定其行为,该语句是 System 命名空间中定义的 Console 类的 method。此语句将导致在屏幕上显示“Hello, World!”的消息。

  7. 最后一行 Console.ReadKey(); 适用于 VS.NET 用户。这会使程序等待按下键并防止在从 Visual Studio .NET 启动程序后屏幕快速运行并关闭。

请注意以下几点:

  1. C# is case sensitive.

  2. 所有陈述和表达式必须以分号(;)结尾。

  3. 程序执行从 Main 方法开始。

  4. 与 Java 不同,程序文件名可以不同于类名。

Compiling and Executing the Program

如果您使用 Visual Studio.Net 来编译和执行 C# 程序,请执行以下步骤:

  1. Start Visual Studio.

  2. 在菜单栏中选择 File → New → Project。

  3. 从中选择 Visual C#,然后选择 Windows。

  4. Choose Console Application.

  5. 指定项目的名称,然后单击“确定”按钮。

  6. 这将在 Solution Explorer 中创建一个新的项目。

  7. 在代码编辑器中编写代码。

  8. 单击“运行”按钮或按 F5 键可执行项目。出现一个命令提示符窗口,其中包含行 Hello World。

您可以通过使用命令行(而不是 Visual Studio IDE)来编译 C# 程序:

  1. 打开文本编辑器并添加上述代码。

  2. 将文件保存为 helloworld.cs

  3. 打开命令提示符工具并转到已保存文件所在的目录中。

  4. 键入 csc helloworld.cs 并按 Enter 键来编译您的代码。

  5. 如果您的代码中没有错误,则命令提示符会将您带到下一行,并生成 helloworld.exe 可执行文件。

  6. 键入 helloworld 可执行程序。

  7. 您会看到输出 Hello World 打印在屏幕上。

C

C# 是一种面向对象编程语言。面向对象编程方法中,程序由各种通过操作相互交互的对象组成。对象可能采取的操作称为方法。相同种类对象被认为具有相同类型或属于同一类。

例如,我们考虑一个 Rectangle 对象。它具有诸如长度和宽度的属性。根据设计,它可能需要接受这些属性值、计算面积并显示详细信息的方法。

我们来看看 Rectangle 类的实现,并讨论 C# 基本语法 −

using System;

namespace RectangleApplication {

   class Rectangle {
      // member variables
      double length;
      double width;

      public void Acceptdetails() {
         length = 4.5;
         width = 3.5;
      }

      public double GetArea() {
         return length * width;
      }

      public void Display() {
         Console.WriteLine("Length: {0}", length);
         Console.WriteLine("Width: {0}", width);
         Console.WriteLine("Area: {0}", GetArea());
      }
   }

   class ExecuteRectangle {

      static void Main(string[] args) {
         Rectangle r = new Rectangle();
         r.Acceptdetails();
         r.Display();
         Console.ReadLine();
      }
   }
}

编译并执行上述代码后,将产生以下结果 −

Length: 4.5
Width: 3.5
Area: 15.75

The using Keyword

任何 C# 程序中的第一条语句为

using System;

using 关键字用于将命名空间包括在程序中。程序可以包括多个 using 语句。

The class Keyword

class 关键字用于声明类。

Comments in C

注释用于解释代码。编译器会忽略注释条目。C# 程序中的多行注释以 /* 开始,并以 */ 结束,如下所示 −

/* This program demonstrates
The basic syntax of C# programming
Language */

单行注释由“//”符号指示。例如:

}//end class Rectangle

Member Variables

变量是类的属性或数据成员,用于存储数据。在前述程序中,Rectangle 类有两个名为 length 和 width 的成员变量。

Member Functions

函数是执行特定任务的语句集。类的成员函数在类中声明。示例类 Rectangle 包含三个成员函数:AcceptDetails、GetArea 和 Display。

Instantiating a Class

在前述程序中,ExecuteRectangle 类包含 Main() 方法,并实例化 Rectangle 类。

Identifiers

标识符是一个用于识别类、变量、函数或任何其他用户定义项的名称。在 C# 中命名类的基本规则如下 −

  1. 名称必须以字母开头,后面可以跟一系列字母、数字 (0 - 9) 或下划线。标识符中的第一个字符不能是数字。

  2. 它不能包含任何内嵌空格或符号,如?-+!@ #% ^ & * ( ) [ ] { } . ; : " ' / 和 \。但是,可以使用下划线 ( _ )。

  3. 它不应该是 C# 关键字。

C

关键字是预先定义为 C# 编译器的保留字。这些关键字不能用作标识符。但是,如果您想将这些关键字用作标识符,您可以在关键字前加上 @ 字符。

在 C# 中,一些标识符在代码上下文中具有特殊含义,例如 get 和 set 被称为上下文关键字。

下表列出了 C# 中的保留关键字和上下文关键字 -

Reserved Keywords

abstract

as

base

bool

break

byte

case

catch

char

checked

class

const

continue

decimal

default

delegate

do

double

else

enum

event

explicit

extern

false

finally

fixed

float

for

foreach

goto

if

implicit

in

in (generic modifier)

int

interface

internal

is

lock

long

namespace

new

null

object

operator

out

out (generic modifier)

override

params

private

protected

public

readonly

ref

return

sbyte

sealed

short

sizeof

stackalloc

static

string

struct

switch

this

throw

true

try

typeof

uint

ulong

unchecked

unsafe

ushort

using

virtual

void

volatile

while

Contextual Keywords

add

alias

ascending

descending

dynamic

from

get

global

group

into

join

let

orderby

partial (type)

partial (method)

remove

select

set

C

C# 中的变量被归类为以下类型 −

  1. Value types

  2. Reference types

  3. Pointer types

Value Type

值类型变量可以直接被赋值。它们来自类 * System.ValueType*。

值类型直接包含数据。一些示例是 int, char, and float ,它分别存储数字、字母和浮点数。当你声明一个 int 类型时,系统会分配内存来存储该值。

下表列出了 C# 2010 中可用的值类型 −

Type

Represents

Range

Default Value

bool

Boolean value

True or False

False

byte

8-bit unsigned integer

0 to 255

0

char

16-bit Unicode character

U +0000 到 U +ffff

'\0'

decimal

28-29 位有效数字的 128 位精确十进制值

(-7.9 x 1028 到 7.9 x 1028) / 100 到 28

0.0M

double

64 位双精度浮点数类型

(/-)5.0 x 10-324 到 (/-)1.7 x 10308

0.0D

float

32 位单精度浮点数类型

-3.4 x 1038 到 + 3.4 x 1038

0.0F

int

32-bit signed integer type

-2,147,483,648 to 2,147,483,647

0

long

64-bit signed integer type

-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807

0L

sbyte

8-bit signed integer type

-128 to 127

0

short

16-bit signed integer type

-32,768 to 32,767

0

uint

32-bit unsigned integer type

0 to 4,294,967,295

0

ulong

64-bit unsigned integer type

0 to 18,446,744,073,709,551,615

0

ushort

16-bit unsigned integer type

0 to 65,535

0

要获取特定平台上某个类型或变量的确切大小,可以使用 sizeof 方法。表达式 sizeof(type) 会产生对象或类型在字节中的存储大小。以下是一个在任何机器上获取 int 类型大小的示例 −

using System;

namespace DataTypeApplication {

   class Program {

      static void Main(string[] args) {
         Console.WriteLine("Size of int: {0}", sizeof(int));
         Console.ReadLine();
      }
   }
}

编译并执行上述代码后,将产生以下结果 −

Size of int: 4

Reference Type

引用类型不包含存储在变量中的实际数据,但它们包含对变量的引用。

换句话说,它们引用一个内存位置。引用类型可以使用多个变量引用内存位置。如果内存位置中的数据被其中一个变量更改,则另一个变量会自动反映此值更改。 built-in 引用类型的示例包括: objectdynamic,string

Object Type

Object Type 是 C# 公共类型系统 (CTS) 中所有数据类型的终极基类。对象是 System.Object 类的一个别名。对象类型可以被赋值任何其他类型、值类型、引用类型、预定义类型或用户定义类型的值。但是,在赋值之前,它需要类型转换。

当值类型转换为对象类型时,称为 boxing ,另一方面,当对象类型转换为值类型时,称为 unboxing

object obj;
obj = 100; // this is boxing

Dynamic Type

可以在 dynamic 数据类型变量中存储任何类型的值。这些类型变量的类型检查在运行时进行。

声明 dynamic 类型的语法是 −

dynamic <variable_name> = value;

例如,

dynamic d = 20;

动态类型类似于对象类型,但对象类型变量的类型检查发生在编译时,而动态类型变量的类型检查发生在运行时。

String Type

String Type 允许你将任意字符串值赋予变量。字符串类型是 System.String 类的别名。它源自对象类型。字符串类型的变量的值可以使用两种形式的字符串文字进行赋值:quoted 和 @quoted。

例如,

String str = "Tutorials Point";

@quoted 字符串文字如下所示:

@"Tutorials Point";

用户定义的引用类型是:类、接口或委托。我们将在后面的章节中讨论这些类型。

Pointer Type

指针类型变量存储另一种类型内存地址。C# 中的指针具有与 C 或 C++ 中指针相同的功能。

声明指针类型的语法是:

type* identifier;

例如,

char* cptr;
int* iptr;

我们将在“不安全代码”一章中讨论指针类型。

C

类型转换是将一种数据类型转换为另一种类型。它也被称为强制类型转换。在 C# 中,类型转换有两种形式 −

  1. Implicit type conversion − 这些转换由 C# 以类型安全的方式执行。例如,是从较小的整数类型到较大的整数类型的转换,以及从派生类到基类的转换。

  2. Explicit type conversion − 这些转换是用户使用预定义函数显式完成的。显式转换需要一个强制转换运算符。

以下示例显示了一个显式类型转换 −

using System;

namespace TypeConversionApplication {

   class ExplicitConversion {

      static void Main(string[] args) {
         double d = 5673.74;
         int i;

         // cast double to int.
         i = (int)d;
         Console.WriteLine(i);
         Console.ReadKey();
      }
   }
}

编译并执行上述代码后,将产生以下结果 −

5673

C

C# 提供以下内置类型转换方法:

Sr.No.

Methods & Description

1

ToBoolean 尽可能将类型转换为布尔值。

2

ToByte 将类型转换为字节。

3

ToChar 尽可能将类型转换为单个 Unicode 字符。

4

ToDateTime 将类型(整数或字符串类型)转换为日期时间结构。

5

ToDecimal 将浮点或整数类型转换为十进制类型。

6

ToDouble 将类型转换为双精度类型。

7

ToInt16 将类型转换为 16 位整数。

8

ToInt32 将类型转换为 32 位整数。

9

ToInt64 将类型转换为 64 位整数。

10

ToSbyte 将类型转换为有符号字节类型。

11

ToSingle 将类型转换为小浮点数。

12

ToString 将类型转换为字符串。

13

ToType 将类型转换为指定类型。

14

ToUInt16 将类型转换为无符号整数类型。

15

ToUInt32 将类型转换为无符号长整型。

16

ToUInt64 将类型转换为无符号大整数。

以下示例将各种值类型转换为字符串类型:

using System;

namespace TypeConversionApplication {

   class StringConversion {

      static void Main(string[] args) {
         int i = 75;
         float f = 53.005f;
         double d = 2345.7652;
         bool b = true;

         Console.WriteLine(i.ToString());
         Console.WriteLine(f.ToString());
         Console.WriteLine(d.ToString());
         Console.WriteLine(b.ToString());
         Console.ReadKey();

      }
   }
}

编译并执行上述代码后,将产生以下结果 −

75
53.005
2345.7652
True

C

变量不过是为我们程序可以操作的存储区域指定的名称。C# 中的每个变量都有一个特定的类型,它决定了该变量的内存大小和布局,该内存中可以存储的值范围,以及可以对该变量应用的操作集。

C# 中提供基本值类型可分为以下几大类:

Type

Example

Integral types

sbyte、byte、short、ushort、int、uint、long、ulong 和 char

Floating point types

float and double

Decimal types

decimal

Boolean types

true 或 false 值(已分配)

Nullable types

Nullable data types

C# 还允许定义其他变量值类型,如 enum ,以及变量引用类型,如 ,我们将在后续章节中予以介绍。

Defining Variables

C# 中的变量定义语法如下:

<data_type> <variable_list>;

此处,data_type 必须是有效的 C# 数据类型,包括 char、int、float、double 或任何用户定义的数据类型,而 variable_list 可能由一个或多个用逗号分隔的标识符名称组成。

这里显示了一些有效的变量定义:

int i, j, k;
char c, ch;
float f, salary;
double d;

您可以初始化变量,如下所示:

int i = 100;

Initializing Variables

变量用等号后跟一个常量表达式进行初始化(赋值)。初始化的一般形式为:

variable_name = value;

变量可以在其声明中初始化。初始化程序由等号后跟常量表达式组成,如下所示:

<data_type> <variable_name> = value;

一些示例:

int d = 3, f = 5;    /* initializing d and f. */
byte z = 22;         /* initializes z. */
double pi = 3.14159; /* declares an approximation of pi. */
char x = 'x';        /* the variable x has the value 'x'. */

最好对变量进行适当初始化,否则程序有时可能会产生意外结果。

以下示例使用各种类型的变量:

using System;

namespace VariableDefinition {

   class Program {

      static void Main(string[] args) {
         short a;
         int b ;
         double c;

         /* actual initialization */
         a = 10;
         b = 20;
         c = a + b;
         Console.WriteLine("a = {0}, b = {1}, c = {2}", a, b, c);
         Console.ReadLine();
      }
   }
}

编译并执行上述代码后,将产生以下结果 −

a = 10, b = 20, c = 30

Accepting Values from User

Console 类在 System 命名空间中提供了一个名为 ReadLine() 的函数,用于接受用户的输入并将其存储到一个变量中。

例如,

int num;
num = Convert.ToInt32(Console.ReadLine());

函数 Convert.ToInt32() 将用户输入的数据转换为 int 数据类型,因为 Console.ReadLine() 接受的格式为字符串。

Lvalue and Rvalue Expressions in C

C# 中有两种表达式:

  1. lvalue - 一个是 lvalue 的表达式可以出现在赋值运算的左边或右边。

  2. rvalue - 一个是 rvalue 的表达式只能出现在赋值运算的右边,不能出现在左边。

变量是 lvalue,因此它们可以出现在赋值运算的左边。数字文本是 rvalue,因此它们不能被赋值,不能出现在赋值运算的左边。以下是一个有效的 C# 语句:

int g = 20;

但是,以下不是有效的语句,将导致编译时错误:

10 = 20;

C

常量是指程序在执行期间可能不会更改的固定值。这些固定值也称为字面量。常量可以是任何基本数据类型,例如整型常量、浮点常量、字符常量或字符串字面量,还可以是枚举常量。

常量与普通变量一样对待,只是它们的定义后不能再修改其值。

Integer Literals

整型字面量可以是十进制常量或十六进制常量。前缀指定基数或进制:十进制为 0x 或 0X,无前缀表示十进制。

整数文本还可以有一个后缀,它是 U 和 L 的组合,分别代表无符号和长。后缀可以是大写或小写,并且可以按任何顺序排列。

以下是整数文本的一些示例:

212         /* Legal */
215u        /* Legal */
0xFeeL      /* Legal */

以下是一些不同类型整数文本的其他示例:

85         /* decimal */
0x4b       /* hexadecimal */
30         /* int */
30u        /* unsigned int */
30l        /* long */
30ul       /* unsigned long */

Floating-point Literals

浮点文本具有整数部分、小数点、小数部分和指数部分。你可以以十进制形式或指数形式表示浮点数文本。

以下是浮点数文本的一些示例:

3.14159       /* Legal */
314159E-5F    /* Legal */
510E          /* Illegal: incomplete exponent */
210f          /* Illegal: no decimal or exponent */
.e55          /* Illegal: missing integer or fraction */

以十进制形式表示时,必须包括小数点、指数或两者;以指数形式表示时,必须包括整数部分、小数部分或两者。带符号的指数由 e 或 E 引入。

Character Constants

字符字面量用单引号括起来。例如,“x” 存储在 char 类型的一个简单变量中。字符字面量可以是普通字符(例如 “x”)、转义序列(例如“\t”)或通用字符(例如“\u02C0”)。

C# 中当某些字符前加反斜杠时,它们具有特殊含义,并且用于表示换行符 (\n) 或制表符 (\t)。以下列出了部分这样的转义序列代码 -

Escape sequence

Meaning

|\ character

\'

' character

\"

" character

\?

? character

\a

Alert or bell

\b

Backspace

\f

Form feed

\n

Newline

\r

Carriage return

\t

Horizontal tab

\v

Vertical tab

\xhh . . .

以下示例演示了一些转义序列字符 -

using System;

namespace EscapeChar {

   class Program {

      static void Main(string[] args) {
         Console.WriteLine("Hello\tWorld\n\n");
         Console.ReadLine();
      }
   }
}

编译并执行上述代码后,将产生以下结果 −

Hello   World

String Literals

字符串字面量或常量用双引号 "" 或 @"" 括起来。字符串包含与字符字面量类似的字符:普通字符、转义序列和通用字符。

可以使用字符串字面量并使用空格分隔各部分,将长代码行分隔为多行。

以下是字符串文本的一些示例。所有这三种形式都是相同的字符串。

"hello, dear"
"hello, \
dear"
"hello, " "d" "ear"
@"hello dear"

Defining Constants

使用 const 关键字定义常量。定义常量的语法为 -

const <data_type> <constant_name> = value;

以下程序演示在程序中定义并使用常数 -

using System;

namespace DeclaringConstants {

    class Program {

        static void Main(string[] args) {
            const double pi = 3.14159;

            // constant declaration
            double r;
            Console.WriteLine("Enter Radius: ");
            r = Convert.ToDouble(Console.ReadLine());
            double areaCircle = pi * r * r;
            Console.WriteLine("Radius: {0}, Area: {1}", r, areaCircle);
            Console.ReadLine();
        }
    }
}

编译并执行上述代码后,将产生以下结果 −

Enter Radius:
3
Radius: 3, Area: 28.27431

C

运算符是一个符号,它告诉编译器执行特定的数学或逻辑运算。C# 具有丰富的内置运算符且提供了以下类型的运算符 −

  1. Arithmetic Operators

  2. Relational Operators

  3. Logical Operators

  4. Bitwise Operators

  5. Assignment Operators

  6. Misc Operators

本教程逐一说明了算术、关系、逻辑、位运算、赋值和其他运算符。

Arithmetic Operators

下表显示了 C# 所支持的所有算术运算符。假设变量 A 存储 10 而变量 B 存储 20,则 −

Operator

Description

Example

+

Adds two operands

A + B = 30

-

从第一个操作数中减去第二个操作数

A - B = -10

*

Multiplies both operands

A * B = 200

/

Divides numerator by de-numerator

B / A = 2

%

模运算符,整数除法后的余数

B % A = 0

++

增量运算符将整数值增加 1

A++ = 11

 — 

减量运算符将整数值减少 1

A-- = 9

Relational Operators

下表显示了 C# 所支持的所有关系运算符。假设变量 A 存储 10 而变量 B 存储 20,则 −

Operator

Description

Example

==

检查两个操作数的值是否相等,如果相等,则条件变为真。

(A == B) 不为真。

!=

检查两个操作数的值是否相等,如果值不相等,则条件变为真。

(A != B) 为真。

>

检查左操作数的值是否大于右操作数的值,如果大于,则条件变为真。

(A > B) 不为真。

<

检查左操作数的值是否小于右操作数的值,如果小于,则条件变为真。

(A < B) 为真。

>=

检查左操作数的值是否大于或等于右操作数的值,如果大于或等于,则条件变为真。

(A >= B) 不为真。

检查左操作数的值是否小于或等于右操作数的值,如果小于或等于,则条件变为真。

(A ⇐ B) 为真。

Logical Operators

下表显示了 C# 所支持的所有逻辑运算符。假设变量 A 存储布尔值 true 而变量 B 存储布尔值 false,则 −

Operator

Description

Example

&&

称为逻辑 AND 运算符。如果两个操作数都是非零,则条件变为真。

(A && B) 为假。

称为逻辑 OR 运算符。如果两个操作数中的任何一个是非零,则条件变为真。

(A

B) is true.

!

称为逻辑 NOT 运算符。用于反转操作数的逻辑状态。如果条件为 true,则逻辑 NOT 运算符会将其变为 false。

Bitwise Operators

位运算符对位操作,并执行逐位操作。&、| 和 ^ 的真值表如下 −

p

q

p &

p

p ^ q

0

0

0

0

0

0

1

0

1

1

1

1

1

1

0

1

0

0

假设 A = 60;B = 13;那么在二进制格式中,它们如下 −

A = 0011 1100

B = 0000 1101

A&B = 0000 1100

A|B = 0011 1101

A^B = 0011 0001

~A  = 1100 0011

The Bitwise operators supported by C# are listed in the following table. Assume variable A holds 60 and variable B holds 13, then −

link:../csharp/csharp_bitwise_operators.html[Show Examples]
[%autowidth]
|===

|Operator|Description|Example
|&|Binary AND Operator copies a bit to the result if it exists in both operands.|(A & B) = 12, which is 0000 1100
|||Binary OR Operator copies a bit if it exists in either operand.|(A | B) = 61, which is 0011 1101
|^|Binary XOR Operator copies the bit if it is set in one operand but not both.|(A ^ B) = 49, which is 0011 0001
|~|Binary Ones Complement Operator is unary and has the effect of 'flipping' bits.|(~A ) = -61, which is 1100 0011 in 2's complement due to a signed binary number.
|<<|Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand.|A << 2 = 240, which is 1111 0000
|>>|Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand.|A >> 2 = 15, which is 0000 1111
|===


=== Assignment Operators

There are following assignment operators supported by C# −

link:../csharp/csharp_assignment_operators.html[Show Examples]
[%autowidth]
|===

|Operator|Description|Example
|=|Simple assignment operator, Assigns values from right side operands to left side operand|C = A + B assigns value of A + B into C
|+=|Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand|C += A is equivalent to C = C + A
|-=|Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand|C -= A is equivalent to C = C - A
|*=|Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand|C *= A is equivalent to C = C * A
|/=|Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand|C /= A is equivalent to C = C / A
|%=|Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand|C %= A is equivalent to C = C % A
|<<=|Left shift AND assignment operator|C <<= 2 is same as C = C << 2
|>>=|Right shift AND assignment operator|C >>= 2 is same as C = C >> 2
|&=|Bitwise AND assignment operator|C &= 2 is same as C = C & 2
|^=|bitwise exclusive OR and assignment operator|C ^= 2 is same as C = C ^ 2
||=|bitwise inclusive OR and assignment operator|C |= 2 is same as C = C | 2
|===


=== Miscellaneous Operators

There are few other important operators including *sizeof, typeof* and *? :* supported by C#.

link:../csharp/csharp_misc_operators.html[Show Examples]
[%autowidth]
|===

|Operator|Description|Example
|sizeof()|Returns the size of a data type.|sizeof(int), returns 4.
|typeof()|Returns the type of a class.|typeof(StreamReader);
|&|Returns the address of an variable.|&a; returns actual address of the variable.
|*|Pointer to a variable.|*a; creates pointer named 'a' to a variable.
|? :|Conditional Expression|If Condition is true ? Then value X : Otherwise value Y
|is|Determines whether an object is of a certain type.|If( Ford is Car) // checks if Ford is an object of the Car class.
|as|Cast without raising an exception if the cast fails.|Object obj = new StringReader("Hello"); StringReader r = obj as StringReader;

|===


=== Operator Precedence in C

Operator precedence determines the grouping of terms in an expression. This affects evaluation of an expression. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator.

For example x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has higher precedence than +, so the first evaluation takes place for 3*2 and then 7 is added into it.

Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators are evaluated first.

link:../csharp/csharp_operators_precedence.html[Show Examples]
[%autowidth]
|===

|Category|Operator|Associativity
|Postfix|() [] -> . ++ - -|Left to right
|Unary|+ - ! ~ ++ - - (type)* & sizeof|Right to left
|Multiplicative|* / %|Left to right
|Additive|+ -|Left to right
|Shift|<< >>|Left to right
|Relational|< <= > >=|Left to right
|Equality|== !=|Left to right
|Bitwise AND|&|Left to right
|Bitwise XOR|^|Left to right
|Bitwise OR|||Left to right
|Logical AND|&&|Left to right
|Logical OR||||Left to right
|Conditional|?:|Right to left
|Assignment|= += -= *= /= %=>>= <<= &= ^= |=|Right to left
|Comma|,|Left to right
|===


== C

Decision making structures requires the programmer to specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

Following is the general form of a typical decision making structure found in most of the programming languages −

image::https://www.iokays.com/tutorialspoint/csharp/_images/decision_making.jpg[Decision making statements in C#]

C# provides following types of decision making statements. Click the following links to check their detail.
[%autowidth]
|===

|Sr.No.|Statement & Description
|1|link:../csharp/if_statement_in_csharp.html[if statement]An *if statement* consists of a boolean expression followed by one or more statements.


|2|link:../csharp/if_else_statement_in_csharp.html[if...else statement]An *if statement* can be followed by an optional *else statement*, which executes when the boolean expression is false.


|3|link:../csharp/nested_if_statements_in_csharp.html[nested if statements]You can use one *if* or *else if* statement inside another *if* or *else if* statement(s).


|4|link:../csharp/switch_statement_in_csharp.html[switch statement]A *switch* statement allows a variable to be tested for equality against a list of values.


|5|link:../csharp/nested_switch_statements_in_csharp.html[nested switch statements]You can use one *switch* statement inside another *switch * statement(s).


|===


=== The ? : Operator

We have covered *conditional operator ? :* in previous chapter which can be used to replace *if...else* statements. It has the following general form −

[source]

Exp1 ? Exp2 : Exp3;

Where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.

The value of a ? expression is determined as follows: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ? expression. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.


== C

There may be a situation, when you need to execute a block of code several number of times. In general, the statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or a group of statements multiple times and following is the general from of a loop statement in most of the programming languages −

image::https://www.iokays.com/tutorialspoint/csharp/_images/loop_architecture.jpg[Loop Architecture]

C# provides following types of loop to handle looping requirements. Click the following links to check their detail.
[%autowidth]
|===

|Sr.No.|Loop Type & Description
|1|link:../csharp/csharp_while_loop.html[while loop]It repeats a statement or a group of statements while a given condition is true. It tests the condition before executing the loop body.


|2|link:../csharp/csharp_for_loop.html[for loop]It executes a sequence of statements multiple times and abbreviates the code that manages the loop variable.


|3|link:../csharp/csharp_do_while_loop.html[do...while loop]It is similar to a while statement, except that it tests the condition at the end of the loop body


|4|link:../csharp/csharp_nested_loops.html[nested loops]You can use one or more loop inside any another while, for or do..while loop.


|===


=== Loop Control Statements

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

C# provides the following control statements. Click the following links to check their details.
[%autowidth]
|===

|Sr.No.|Control Statement & Description
|1|link:../csharp/csharp_break_statement.html[break statement]Terminates the *loop* or *switch* statement and transfers execution to the statement immediately following the loop or switch.


|2|link:../csharp/csharp_continue_statement.html[continue statement]Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.


|===


=== Infinite Loop

A loop becomes infinite loop if a condition never becomes false. The *for* loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, you can make an endless loop by leaving the conditional expression empty.


==== Example

[source]

using System;

namespace Loops {

class Program {
      static void Main(string[] args) {
         for (; ; ) {
            Console.WriteLine("Hey! I am Trapped");
         }
      }
   }
}
When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but programmers more commonly use the for(;;) construct to signify an infinite loop.


== C

*Encapsulation* is defined 'as the process of enclosing one or more items within a physical or logical package'. Encapsulation, in object oriented programming methodology, prevents access to implementation details.

Abstraction and encapsulation are related features in object oriented programming. Abstraction allows making relevant information visible and encapsulation enables a programmer to implement the desired level of abstraction.

Encapsulation is implemented by using *access specifiers*. An *access specifier* defines the scope and visibility of a class member. C# supports the following access specifiers −

. Public

. Private

. Protected

. Internal

. Protected internal


=== Public Access Specifier

Public access specifier allows a class to expose its member variables and member functions to other functions and objects. Any public member can be accessed from outside the class.

The following example illustrates this −

[source]

using System;

namespace RectangleApplication {

class Rectangle {
   //member variables
   public double length;
   public double width;
public double GetArea() {
   return length * width;
}
   public void Display() {
      Console.WriteLine("Length: {0}", length);
      Console.WriteLine("Width: {0}", width);
      Console.WriteLine("Area: {0}", GetArea());
   }
}//end class Rectangle
   class ExecuteRectangle {
      static void Main(string[] args) {
         Rectangle r = new Rectangle();
         r.length = 4.5;
         r.width = 3.5;
         r.Display();
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Length: 4.5 Width: 3.5 Area: 15.75

In the preceding example, the member variables length and width are declared *public*, so they can be accessed from the function Main() using an instance of the Rectangle class, named *r*.

The member function Display() and GetArea() can also access these variables directly without using any instance of the class.

The member functions Display() is also declared *public*, so it can also be accessed from Main() using an instance of the Rectangle class, named *r*.


=== Private Access Specifier

Private access specifier allows a class to hide its member variables and member functions from other functions and objects. Only functions of the same class can access its private members. Even an instance of a class cannot access its private members.

The following example illustrates this −

[source]

using System;

namespace RectangleApplication {

class Rectangle {
   //member variables
   private double length;
   private double width;
public void Acceptdetails() {
   Console.WriteLine("Enter Length: ");
   length = Convert.ToDouble(Console.ReadLine());
   Console.WriteLine("Enter Width: ");
   width = Convert.ToDouble(Console.ReadLine());
}
public double GetArea() {
   return length * width;
}
   public void Display() {
      Console.WriteLine("Length: {0}", length);
      Console.WriteLine("Width: {0}", width);
      Console.WriteLine("Area: {0}", GetArea());
   }
}//end class Rectangle
   class ExecuteRectangle {
      static void Main(string[] args) {
         Rectangle r = new Rectangle();
         r.Acceptdetails();
         r.Display();
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Enter Length: 4.4 Enter Width: 3.3 Length: 4.4 Width: 3.3 Area: 14.52

In the preceding example, the member variables length and width are declared *private*, so they cannot be accessed from the function Main(). The member functions AcceptDetails() and Display() can access these variables. Since the member functions AcceptDetails() and Display() are declared *public*, they can be accessed from Main() using an instance of the Rectangle class, named *r*.


=== Protected Access Specifier

Protected access specifier allows a child class to access the member variables and member functions of its base class. This way it helps in implementing inheritance. We will discuss this in more details in the inheritance chapter.


=== Internal Access Specifier

Internal access specifier allows a class to expose its member variables and member functions to other functions and objects in the current assembly. In other words, any member with internal access specifier can be accessed from any class or method defined within the application in which the member is defined.

The following program illustrates this −

[source]

using System;

namespace RectangleApplication {

class Rectangle {
   //member variables
   internal double length;
   internal double width;
double GetArea() {
   return length * width;
}
   public void Display() {
      Console.WriteLine("Length: {0}", length);
      Console.WriteLine("Width: {0}", width);
      Console.WriteLine("Area: {0}", GetArea());
   }
}//end class Rectangle
   class ExecuteRectangle {
      static void Main(string[] args) {
         Rectangle r = new Rectangle();
         r.length = 4.5;
         r.width = 3.5;
         r.Display();
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Length: 4.5 Width: 3.5 Area: 15.75

In the preceding example, notice that the member function GetArea() is not declared with any access specifier. Then what would be the default access specifier of a class member if we don't mention any? It is *private*.


=== Protected Internal Access Specifier

The protected internal access specifier allows a class to hide its member variables and member functions from other class objects and functions, except a child class within the same application. This is also used while implementing inheritance.


== C

A method is a group of statements that together perform a task. Every C# program has at least one class with a method named Main.

To use a method, you need to −

. Define the method

. Call the method


=== Defining Methods in C

When you define a method, you basically declare the elements of its structure. The syntax for defining a method in C# is as follows −

[source]

<Access Specifier> <Return Type> <Method Name>(Parameter List) { Method Body }

Following are the various elements of a method −

. *Access Specifier* − This determines the visibility of a variable or a method from another class.


. *Return type* − A method may return a value. The return type is the data type of the value the method returns. If the method is not returning any values, then the return type is *void*.


. *Method name* − Method name is a unique identifier and it is case sensitive. It cannot be same as any other identifier declared in the class.


. *Parameter list* − Enclosed between parentheses, the parameters are used to pass and receive data from a method. The parameter list refers to the type, order, and number of the parameters of a method. Parameters are optional; that is, a method may contain no parameters.


. *Method body* − This contains the set of instructions needed to complete the required activity.



=== Example

Following code snippet shows a function FindMax that takes two integer values and returns the larger of the two. It has public access specifier, so it can be accessed from outside the class using an instance of the class.

[source]

class NumberManipulator {

public int FindMax(int num1, int num2) {
   /* local variable declaration */
   int result;
if (num1 > num2)
   result = num1;
else
   result = num2;
      return result;
   }
   ...
}
=== Calling Methods in C

You can call a method using the name of the method. The following example illustrates this −

[source]

using System;

namespace CalculatorApplication {

class NumberManipulator {
public int FindMax(int num1, int num2) {
   /* local variable declaration */
   int result;
   if (num1 > num2)
      result = num1;
   else
      result = num2;
   return result;
}
static void Main(string[] args) {
   /* local variable definition */
   int a = 100;
   int b = 200;
   int ret;
   NumberManipulator n = new NumberManipulator();
         //calling the FindMax method
         ret = n.FindMax(a, b);
         Console.WriteLine("Max value is : {0}", ret );
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Max value is : 200

You can also call public method from other classes by using the instance of the class. For example, the method FindMax belongs to the NumberManipulator class, you can call it from another class Test.

[source]

using System;

namespace CalculatorApplication {

class NumberManipulator {
public int FindMax(int num1, int num2) {
   /* local variable declaration */
   int result;
if(num1 > num2)
   result = num1;
else
   result = num2;
      return result;
   }
}
class Test {
static void Main(string[] args) {
   /* local variable definition */
   int a = 100;
   int b = 200;
   int ret;
   NumberManipulator n = new NumberManipulator();
         //calling the FindMax method
         ret = n.FindMax(a, b);
         Console.WriteLine("Max value is : {0}", ret );
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Max value is : 200

=== Recursive Method Call

A method can call itself. This is known as *recursion*. Following is an example that calculates factorial for a given number using a recursive function −

[source]

using System;

namespace CalculatorApplication {

class NumberManipulator {
public int factorial(int num) {
   /* local variable declaration */
   int result;
   if (num == 1) {
      return 1;
   }
   else {
      result = factorial(num - 1) * num;
      return result;
   }
}
      static void Main(string[] args) {
         NumberManipulator n = new NumberManipulator();
         //calling the factorial method {0}", n.factorial(6));
         Console.WriteLine("Factorial of 7 is : {0}", n.factorial(7));
         Console.WriteLine("Factorial of 8 is : {0}", n.factorial(8));
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Factorial of 6 is: 720 Factorial of 7 is: 5040 Factorial of 8 is: 40320

=== Passing Parameters to a Method

When method with parameters is called, you need to pass the parameters to the method. There are three ways that parameters can be passed to a method −
[%autowidth]
|===

|Sr.No.|Mechanism & Description
|1|link:../csharp/csharp_value_parameters.html[Value parameters]This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.


|2|link:../csharp/csharp_reference_parameters.html[Reference parameters]This method copies the reference to the memory location of an argument into the formal parameter. This means that changes made to the parameter affect the argument.


|3|link:../csharp/csharp_output_parameters.html[Output parameters]This method helps in returning more than one value.


|===


== C

C# provides a special data types, the *nullable* types, to which you can assign normal range of values as well as null values.

For example, you can store any value from -2,147,483,648 to 2,147,483,647 or null in a Nullable<Int32> variable. Similarly, you can assign true, false, or null in a Nullable<bool> variable. Syntax for declaring a *nullable* type is as follows −

[source]

< data_type> ? <variable_name> = null;

The following example demonstrates use of nullable data types −

[source]

using System;

namespace CalculatorApplication {

class NullablesAtShow {
static void Main(string[] args) {
   int? num1 = null;
   int? num2 = 45;
   double? num3 = new double?();
   double? num4 = 3.14157;
bool? boolval = new bool?();
         // display the values
         Console.WriteLine("Nullables at Show: {0}, {1}, {2}, {3}", num1, num2, num3, num4);
         Console.WriteLine("A Nullable boolean value: {0}", boolval);
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Nullables at Show: , 45, , 3.14157 A Nullable boolean value:

=== The Null Coalescing Operator (??)

The null coalescing operator is used with the nullable value types and reference types. It is used for converting an operand to the type of another nullable (or not) value type operand, where an implicit conversion is possible.

If the value of the first operand is null, then the operator returns the value of the second operand, otherwise it returns the value of the first operand. The following example explains this −

[source]

using System;

namespace CalculatorApplication {

class NullablesAtShow {
      static void Main(string[] args) {
         double? num1 = null;
         double? num2 = 3.14157;
         double num3;
         num3 = num1 ?? 5.34;
         Console.WriteLine(" Value of num3: {0}", num3);
         num3 = num2 ?? 5.34;
         Console.WriteLine(" Value of num3: {0}", num3);
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Value of num3: 5.34 Value of num3: 3.14157

== C

An array stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type stored at contiguous memory locations.

Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index.

All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

image::https://www.iokays.com/tutorialspoint/csharp/_images/arrays.jpg[Arrays in C#]


=== Declaring Arrays

To declare an array in C#, you can use the following syntax −

[source]

datatype[] arrayName;

where,

. datatype is used to specify the type of elements in the array.


. [ ] specifies the rank of the array. The rank specifies the size of the array.


. arrayName specifies the name of the array.


For example,

[source]

double[] balance;

=== Initializing an Array

Declaring an array does not initialize the array in the memory. When the array variable is initialized, you can assign values to the array.

Array is a reference type, so you need to use the *new* keyword to create an instance of the array. For example,

[source]

double[] balance = new double[10];

=== Assigning Values to an Array

You can assign values to individual array elements, by using the index number, like −

[source]

double[] balance = new double[10]; balance[0] = 4500.0;

You can assign values to the array at the time of declaration, as shown −

[source]

double[] balance = { 2340.0, 4523.69, 3421.0};

You can also create and initialize an array, as shown −

[source]

int [] marks = new int[5] { 99, 98, 92, 97, 95};

You may also omit the size of the array, as shown −

[source]

int [] marks = new int[] { 99, 98, 92, 97, 95};

You can copy an array variable into another target array variable. In such case, both the target and source point to the same memory location −

[source]

int [] marks = new int[] { 99, 98, 92, 97, 95}; int[] score = marks;

When you create an array, C# compiler implicitly initializes each array element to a default value depending on the array type. For example, for an int array all elements are initialized to 0.


=== Accessing Array Elements

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example,

[source]

double salary = balance[9];

The following example, demonstrates the above-mentioned concepts declaration, assignment, and accessing arrays −

[source]

using System;

namespace ArrayApplication {

class MyArray {
static void Main(string[] args) {
   int []  n = new int[10]; /* n is an array of 10 integers */
   int i,j;
/* initialize elements of array n */
for ( i = 0; i < 10; i++ ) {
   n[ i ] = i + 100;
}
         /* output each array element's value */
         for (j = 0; j < 10; j++ ) {
            Console.WriteLine("Element[{0}] = {1}", j, n[j]);
         }
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Element[0] = 100 Element[1] = 101 Element[2] = 102 Element[3] = 103 Element[4] = 104 Element[5] = 105 Element[6] = 106 Element[7] = 107 Element[8] = 108 Element[9] = 109

=== Using the foreach Loop

In the previous example, we used a for loop for accessing each array element. You can also use a *foreach* statement to iterate through an array.

[source]

using System;

namespace ArrayApplication {

class MyArray {
static void Main(string[] args) {
   int []  n = new int[10]; /* n is an array of 10 integers */
/* initialize elements of array n */
for ( int i = 0; i < 10; i++ ) {
   n[i] = i + 100;
}
/* output each array element's value */
foreach (int j in n ) {
   int i = j-100;
   Console.WriteLine("Element[{0}] = {1}", i, j);
         }
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Element[0] = 100 Element[1] = 101 Element[2] = 102 Element[3] = 103 Element[4] = 104 Element[5] = 105 Element[6] = 106 Element[7] = 107 Element[8] = 108 Element[9] = 109

=== C

There are following few important concepts related to array which should be clear to a C# programmer −
[%autowidth]
|===

|Sr.No.|Concept & Description
|1|link:../csharp/csharp_multi_dimensional_arrays.html[Multi-dimensional arrays]C# supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array.


|2|link:../csharp/csharp_jagged_arrays.html[Jagged arrays]C# supports multidimensional arrays, which are arrays of arrays.


|3|link:../csharp/csharp_passing_arrays_to_functions.html[Passing arrays to functions]You can pass to the function a pointer to an array by specifying the array's name without an index.


|4|link:../csharp/csharp_param_arrays.html[Param arrays]This is used for passing unknown number of parameters to a function.


|5|link:../csharp/csharp_array_class.html[The Array Class]Defined in System namespace, it is the base class to all arrays, and provides various properties and methods for working with arrays.


|===


== C

In C#, you can use strings as array of characters, However, more common practice is to use the *string* keyword to declare a string variable. The string keyword is an alias for the *System.String* class.


=== Creating a String Object

You can create string object using one of the following methods −

. By assigning a string literal to a String variable


. By using a String class constructor


. By using the string concatenation operator (+)


. By retrieving a property or calling a method that returns a string


. By calling a formatting method to convert a value or an object to its string representation


The following example demonstrates this −

[source]

using System;

namespace StringApplication {

class Program {
static void Main(string[] args) {
   //from string literal and string concatenation
   string fname, lname;
   fname = "Rowan";
   lname = "Atkinson";
char []letters= { 'H', 'e', 'l', 'l','o' };
string [] sarray={ "Hello", "From", "Tutorials", "Point" };
string fullname = fname + lname;
Console.WriteLine("Full Name: {0}", fullname);
//by using string constructor { 'H', 'e', 'l', 'l','o' };
string greetings = new string(letters);
Console.WriteLine("Greetings: {0}", greetings);
//methods returning string { "Hello", "From", "Tutorials", "Point" };
string message = String.Join(" ", sarray);
Console.WriteLine("Message: {0}", message);
         //formatting method to convert a value
         DateTime waiting = new DateTime(2012, 10, 10, 17, 58, 1);
         string chat = String.Format("Message sent at {0:t} on {0:D}", waiting);
         Console.WriteLine("Message: {0}", chat);
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Full Name: RowanAtkinson Greetings: Hello Message: Hello From Tutorials Point Message: Message sent at 5:58 PM on Wednesday, October 10, 2012

=== Properties of the String Class

The String class has the following two properties −
[%autowidth]
|===

|Sr.No.|Property & Description
|1|*Chars*
Gets the Char object at a specified position in the current String object.

|2|*Length*
Gets the number of characters in the current String object.

|===


=== Methods of the String Class

The String class has numerous methods that help you in working with the string objects. The following table provides some of the most commonly used methods −

You can visit MSDN library for the complete list of methods and String class constructors.


=== Examples

The following example demonstrates some of the methods mentioned above −


==== Comparing Strings

[source]

using System;

namespace StringApplication {

class StringProg {
static void Main(string[] args) {
   string str1 = "This is test";
   string str2 = "This is text";
         if (String.Compare(str1, str2) == 0) {
            Console.WriteLine(str1 + " and " + str2 +  " are equal.");
         } else {
            Console.WriteLine(str1 + " and " + str2 + " are not equal.");
         }
         Console.ReadKey() ;
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

This is test and This is text are not equal.

==== String Contains String

[source]

using System;

namespace StringApplication {

class StringProg {
static void Main(string[] args) {
   string str = "This is test";
         if (str.Contains("test")) {
            Console.WriteLine("The sequence 'test' was found.");
         }
         Console.ReadKey() ;
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

The sequence 'test' was found.

==== Getting a Substring

[source]

using System;

namespace StringApplication {

class StringProg {
      static void Main(string[] args) {
         string str = "Last night I dreamt of San Pedro";
         Console.WriteLine(str);
         string substr = str.Substring(23);
         Console.WriteLine(substr);
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

San Pedro

==== Joining Strings

[source]

using System;

namespace StringApplication {

class StringProg {
static void Main(string[] args) {
   string[] starray = new string[]{"Down the way nights are dark",
      "And the sun shines daily on the mountain top",
      "I took a trip on a sailing ship",
      "And when I reached Jamaica",
      "I made a stop"};
         string str = String.Join("\n", starray);
         Console.WriteLine(str);
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Down the way nights are dark And the sun shines daily on the mountain top I took a trip on a sailing ship And when I reached Jamaica I made a stop

== C

In C#, a structure is a value type data type. It helps you to make a single variable hold related data of various data types. The *struct* keyword is used for creating a structure.

Structures are used to represent a record. Suppose you want to keep track of your books in a library. You might want to track the following attributes about each book −

. Title

. Author

. Subject

. Book ID


=== Defining a Structure

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member for your program.

For example, here is the way you can declare the Book structure −

[source]

struct Books { public string title; public string author; public string subject; public int book_id; };

The following program shows the use of the structure −

[source]

using System;

struct Books { public string title; public string author; public string subject; public int book_id; };

public class testStructure {

public static void Main(string[] args) {
   Books Book1;   /* Declare Book1 of type Book */
   Books Book2;   /* Declare Book2 of type Book */
/* book 1 specification */
Book1.title = "C Programming";
Book1.author = "Nuha Ali";
Book1.subject = "C Programming Tutorial";
Book1.book_id = 6495407;
/* book 2 specification */
Book2.title = "Telecom Billing";
Book2.author = "Zara Ali";
Book2.subject =  "Telecom Billing Tutorial";
Book2.book_id = 6495700;
/* print Book1 info */
Console.WriteLine( "Book 1 title : {0}", Book1.title);
Console.WriteLine("Book 1 author : {0}", Book1.author);
Console.WriteLine("Book 1 subject : {0}", Book1.subject);
Console.WriteLine("Book 1 book_id :{0}", Book1.book_id);
/* print Book2 info */
Console.WriteLine("Book 2 title : {0}", Book2.title);
Console.WriteLine("Book 2 author : {0}", Book2.author);
Console.WriteLine("Book 2 subject : {0}", Book2.subject);
Console.WriteLine("Book 2 book_id : {0}", Book2.book_id);
      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Book 1 title : C Programming Book 1 author : Nuha Ali Book 1 subject : C Programming Tutorial Book 1 book_id : 6495407 Book 2 title : Telecom Billing Book 2 author : Zara Ali Book 2 subject : Telecom Billing Tutorial Book 2 book_id : 6495700

=== Features of C

You have already used a simple structure named Books. Structures in C# are quite different from that in traditional C or C++. The C# structures have the following features −

. Structures can have methods, fields, indexers, properties, operator methods, and events.


. Structures can have defined constructors, but not destructors. However, you cannot define a default constructor for a structure. The default constructor is automatically defined and cannot be changed.


. Unlike classes, structures cannot inherit other structures or classes.


. Structures cannot be used as a base for other structures or classes.


. A structure can implement one or more interfaces.


. Structure members cannot be specified as abstract, virtual, or protected.


. When you create a struct object using the *New* operator, it gets created and the appropriate constructor is called. Unlike classes, structs can be instantiated without using the New operator.


. If the New operator is not used, the fields remain unassigned and the object cannot be used until all the fields are initialized.



=== Class versus Structure

Classes and Structures have the following basic differences −

. classes are reference types and structs are value types

. structures do not support inheritance

. structures cannot have default constructor

In the light of the above discussions, let us rewrite the previous example −

[source]

using System;

struct Books { private string title; private string author; private string subject; private int book_id;

public void getValues(string t, string a, string s, int id) {
   title = t;
   author = a;
   subject = s;
   book_id = id;
}
   public void display() {
      Console.WriteLine("Title : {0}", title);
      Console.WriteLine("Author : {0}", author);
      Console.WriteLine("Subject : {0}", subject);
      Console.WriteLine("Book_id :{0}", book_id);
   }
};

public class testStructure {

public static void Main(string[] args) {
   Books Book1 = new Books();   /* Declare Book1 of type Book */
   Books Book2 = new Books();   /* Declare Book2 of type Book */
/* book 1 specification */
Book1.getValues("C Programming",
"Nuha Ali", "C Programming Tutorial",6495407);
/* book 2 specification */
Book2.getValues("Telecom Billing",
"Zara Ali", "Telecom Billing Tutorial", 6495700);
/* print Book1 info */
Book1.display();
/* print Book2 info */
Book2.display();
      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Title : C Programming Author : Nuha Ali Subject : C Programming Tutorial Book_id : 6495407 Title : Telecom Billing Author : Zara Ali Subject : Telecom Billing Tutorial Book_id : 6495700

== C

An enumeration is a set of named integer constants. An enumerated type is declared using the *enum* keyword.

C# enumerations are value data type. In other words, enumeration contains its own values and cannot inherit or cannot pass inheritance.


=== Declaring enum Variable

The general syntax for declaring an enumeration is −

[source]

enum <enum_name> { enumeration list };

Where,

. The enum_name specifies the enumeration type name.


. The enumeration list is a comma-separated list of identifiers.


Each of the symbols in the enumeration list stands for an integer value, one greater than the symbol that precedes it. By default, the value of the first enumeration symbol is 0. For example −

[source]

enum Days { Sun, Mon, tue, Wed, thu, Fri, Sat };

=== Example

The following example demonstrates use of enum variable −

[source]

using System;

namespace EnumApplication {

class EnumProgram {
   enum Days { Sun, Mon, tue, Wed, thu, Fri, Sat };
      static void Main(string[] args) {
         int WeekdayStart = (int)Days.Mon;
         int WeekdayEnd = (int)Days.Fri;
         Console.WriteLine("Monday: {0}", WeekdayStart);
         Console.WriteLine("Friday: {0}", WeekdayEnd);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Monday: 1 Friday: 5

== C

When you define a class, you define a blueprint for a data type. This does not actually define any data, but it does define what the class name means. That is, what an object of the class consists of and what operations can be performed on that object. Objects are instances of a class. The methods and variables that constitute a class are called members of the class.


=== Defining a Class

A class definition starts with the keyword class followed by the class name; and the class body enclosed by a pair of curly braces. Following is the general form of a class definition −

[source]

<access specifier> class class_name { // member variables <access specifier> <data type> variable1; <access specifier> <data type> variable2; …​ <access specifier> <data type> variableN; // member methods <access specifier> <return type> method1(parameter_list) { // method body } <access specifier> <return type> method2(parameter_list) { // method body } …​ <access specifier> <return type> methodN(parameter_list) { // method body } }

Note −

. Access specifiers specify the access rules for the members as well as the class itself. If not mentioned, then the default access specifier for a class type is *internal*. Default access for the members is *private*.


. Data type specifies the type of variable, and return type specifies the data type of the data the method returns, if any.


. To access the class members, you use the dot (.) operator.


. The dot operator links the name of an object with the name of a member.


The following example illustrates the concepts discussed so far −

[source]

using System;

namespace BoxApplication {

class Box {
   public double length;   // Length of a box
   public double breadth;  // Breadth of a box
   public double height;   // Height of a box
}
class Boxtester {
static void Main(string[] args) {
    Box Box1 = new Box();   // Declare Box1 of type Box
    Box Box2 = new Box();   // Declare Box2 of type Box
    double volume = 0.0;    // Store the volume of a box here
// box 1 specification
Box1.height = 5.0;
Box1.length = 6.0;
Box1.breadth = 7.0;
// box 2 specification
Box2.height = 10.0;
Box2.length = 12.0;
Box2.breadth = 13.0;
// volume of box 1
volume = Box1.height * Box1.length * Box1.breadth;
Console.WriteLine("Volume of Box1 : {0}",  volume);
            // volume of box 2
            volume = Box2.height * Box2.length * Box2.breadth;
            Console.WriteLine("Volume of Box2 : {0}", volume);
            Console.ReadKey();
        }
    }
}
When the above code is compiled and executed, it produces the following result −

[source]

Volume of Box1 : 210 Volume of Box2 : 1560

=== Member Functions and Encapsulation

A member function of a class is a function that has its definition or its prototype within the class definition similar to any other variable. It operates on any object of the class of which it is a member, and has access to all the members of a class for that object.

Member variables are the attributes of an object (from design perspective) and they are kept private to implement encapsulation. These variables can only be accessed using the public member functions.

Let us put above concepts to set and get the value of different class members in a class −

[source]

using System;

namespace BoxApplication {

class Box {
   private double length;   // Length of a box
   private double breadth;  // Breadth of a box
   private double height;   // Height of a box
public void setLength( double len ) {
   length = len;
}
public void setBreadth( double bre ) {
   breadth = bre;
}
   public void setHeight( double hei ) {
      height = hei;
   }
   public double getVolume() {
      return length * breadth * height;
   }
}
class Boxtester {
static void Main(string[] args) {
   Box Box1 = new Box();   // Declare Box1 of type Box
   Box Box2 = new Box();
   double volume;
// Declare Box2 of type Box
// box 1 specification
Box1.setLength(6.0);
Box1.setBreadth(7.0);
Box1.setHeight(5.0);
// box 2 specification
Box2.setLength(12.0);
Box2.setBreadth(13.0);
Box2.setHeight(10.0);
// volume of box 1
volume = Box1.getVolume();
Console.WriteLine("Volume of Box1 : {0}" ,volume);
// volume of box 2
volume = Box2.getVolume();
Console.WriteLine("Volume of Box2 : {0}", volume);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Volume of Box1 : 210 Volume of Box2 : 1560

=== C

A class *constructor* is a special member function of a class that is executed whenever we create new objects of that class.

A constructor has exactly the same name as that of class and it does not have any return type. Following example explains the concept of constructor −

[source]

using System;

namespace LineApplication {

class Line {
   private double length;   // Length of a line
public Line() {
   Console.WriteLine("Object is being created");
}
public void setLength( double len ) {
   length = len;
}
public double getLength() {
   return length;
}
static void Main(string[] args) {
   Line line = new Line();
         // set line length
         line.setLength(6.0);
         Console.WriteLine("Length of line : {0}", line.getLength());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Object is being created Length of line : 6

A *default constructor* does not have any parameter but if you need, a constructor can have parameters. Such constructors are called *parameterized constructors*. This technique helps you to assign initial value to an object at the time of its creation as shown in the following example −

[source]

using System;

namespace LineApplication {

class Line {
   private double length;   // Length of a line
public Line(double len) {  //Parameterized constructor
   Console.WriteLine("Object is being created, length = {0}", len);
   length = len;
}
public void setLength( double len ) {
   length = len;
}
public double getLength() {
   return length;
}
static void Main(string[] args) {
   Line line = new Line(10.0);
   Console.WriteLine("Length of line : {0}", line.getLength());
         // set line length
         line.setLength(6.0);
         Console.WriteLine("Length of line : {0}", line.getLength());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Object is being created, length = 10 Length of line : 10 Length of line : 6

=== C

A *destructor* is a special member function of a class that is executed whenever an object of its class goes out of scope. A *destructor* has exactly the same name as that of the class with a prefixed tilde (~) and it can neither return a value nor can it take any parameters.

Destructor can be very useful for releasing memory resources before exiting the program. Destructors cannot be inherited or overloaded.

Following example explains the concept of destructor −

[source]

using System;

namespace LineApplication {

class Line {
   private double length;   // Length of a line
public Line() {   // constructor
   Console.WriteLine("Object is being created");
}
~Line() {   //destructor
   Console.WriteLine("Object is being deleted");
}
public void setLength( double len ) {
   length = len;
}
public double getLength() {
   return length;
}
static void Main(string[] args) {
   Line line = new Line();
         // set line length
         line.setLength(6.0);
         Console.WriteLine("Length of line : {0}", line.getLength());
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Object is being created Length of line : 6 Object is being deleted

=== Static Members of a C

We can define class members as static using the *static* keyword. When we declare a member of a class as static, it means no matter how many objects of the class are created, there is only one copy of the static member.

The keyword *static* implies that only one instance of the member exists for a class. Static variables are used for defining constants because their values can be retrieved by invoking the class without creating an instance of it. Static variables can be initialized outside the member function or class definition. You can also initialize static variables inside the class definition.

The following example demonstrates the use of *static variables* −

[source]

using System;

namespace StaticVarApplication {

class StaticVar {
   public static int num;
public void count() {
   num++;
}
   public int getNum() {
      return num;
   }
}
class StaticTester {
      static void Main(string[] args) {
         StaticVar s1 = new StaticVar();
         StaticVar s2 = new StaticVar();
         s1.count();
         s1.count();
         s1.count();
         s2.count();
         s2.count();
         s2.count();
         Console.WriteLine("Variable num for s1: {0}", s1.getNum());
         Console.WriteLine("Variable num for s2: {0}", s2.getNum());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Variable num for s1: 6 Variable num for s2: 6

You can also declare a *member function* as *static*. Such functions can access only static variables. The static functions exist even before the object is created. The following example demonstrates the use of *static functions* −

[source]

using System;

namespace StaticVarApplication {

class StaticVar {
   public static int num;
public void count() {
   num++;
}
   public static int getNum() {
      return num;
   }
}
class StaticTester {
      static void Main(string[] args) {
         StaticVar s = new StaticVar();
         s.count();
         s.count();
         s.count();
         Console.WriteLine("Variable num: {0}", StaticVar.getNum());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Variable num: 3

== C

One of the most important concepts in object-oriented programming is inheritance. Inheritance allows us to define a class in terms of another class, which makes it easier to create and maintain an application. This also provides an opportunity to reuse the code functionality and speeds up implementation time.

When creating a class, instead of writing completely new data members and member functions, the programmer can designate that the new class should inherit the members of an existing class. This existing class is called the *base* class, and the new class is referred to as the *derived* class.

The idea of inheritance implements the *IS-A* relationship. For example, mammal *IS A* animal, dog *IS-A* mammal hence dog *IS-A* animal as well, and so on.


=== Base and Derived Classes

A class can be derived from more than one class or interface, which means that it can inherit data and functions from multiple base classes or interfaces.

The syntax used in C# for creating derived classes is as follows −

[source]

<acess-specifier> class <base_class> { …​ }

class <derived_class> : <base_class> { …​ }

Consider a base class Shape and its derived class Rectangle −

[source]

using System;

namespace InheritanceApplication {

class Shape {
public void setWidth(int w) {
   width = w;
}
   public void setHeight(int h) {
      height = h;
   }
   protected int width;
   protected int height;
}
// Derived class
class Rectangle: Shape {
   public int getArea() {
      return (width * height);
   }
}
class RectangleTester {
static void Main(string[] args) {
   Rectangle Rect = new Rectangle();
Rect.setWidth(5);
Rect.setHeight(7);
         // Print the area of the object.
         Console.WriteLine("Total area: {0}",  Rect.getArea());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Total area: 35

=== Initializing Base Class

The derived class inherits the base class member variables and member methods. Therefore the super class object should be created before the subclass is created. You can give instructions for superclass initialization in the member initialization list.

The following program demonstrates this −

[source]

using System;

namespace RectangleApplication {

class Rectangle {
   //member variables
   protected double length;
   protected double width;
public Rectangle(double l, double w) {
   length = l;
   width = w;
}
public double GetArea() {
   return length * width;
}
   public void Display() {
      Console.WriteLine("Length: {0}", length);
      Console.WriteLine("Width: {0}", width);
      Console.WriteLine("Area: {0}", GetArea());
   }
}//end class Rectangle
class Tabletop : Rectangle {
   private double cost;
   public Tabletop(double l, double w) : base(l, w) { }
public double GetCost() {
   double cost;
   cost = GetArea() * 70;
   return cost;
}
   public void Display() {
      base.Display();
      Console.WriteLine("Cost: {0}", GetCost());
   }
}
class ExecuteRectangle {
      static void Main(string[] args) {
         Tabletop t = new Tabletop(4.5, 7.5);
         t.Display();
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Length: 4.5 Width: 7.5 Area: 33.75 Cost: 2362.5

=== Multiple Inheritance in C

*C# does not support multiple inheritance*. However, you can use interfaces to implement multiple inheritance. The following program demonstrates this −

[source]

using System;

namespace InheritanceApplication {

class Shape {
public void setWidth(int w) {
   width = w;
}
   public void setHeight(int h) {
      height = h;
   }
   protected int width;
   protected int height;
}
// Base class PaintCost
public interface PaintCost {
   int getCost(int area);
}
// Derived class
class Rectangle : Shape, PaintCost {
public int getArea() {
   return (width * height);
}
   public int getCost(int area) {
      return area * 70;
   }
}
class RectangleTester {
static void Main(string[] args) {
   Rectangle Rect = new Rectangle();
   int area;
   Rect.setWidth(5);
   Rect.setHeight(7);
   area = Rect.getArea();
         // Print the area of the object.
         Console.WriteLine("Total area: {0}",  Rect.getArea());
         Console.WriteLine("Total paint cost: ${0}" , Rect.getCost(area));
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Total area: 35 Total paint cost: $2450

== C

The word *polymorphism* means having many forms. In object-oriented programming paradigm, polymorphism is often expressed as 'one interface, multiple functions'.

Polymorphism can be static or dynamic. In *static polymorphism*, the response to a function is determined at the compile time. In *dynamic polymorphism*, it is decided at run-time.


=== Static Polymorphism

The mechanism of linking a function with an object during compile time is called early binding. It is also called static binding. C# provides two techniques to implement static polymorphism. They are −

. Function overloading

. Operator overloading

We discuss operator overloading in next chapter.


=== Function Overloading

You can have multiple definitions for the same function name in the same scope. The definition of the function must differ from each other by the types and/or the number of arguments in the argument list. You cannot overload function declarations that differ only by return type.

The following example shows using function *print()* to print different data types −

[source]

using System;

namespace PolymorphismApplication {

class Printdata {
void print(int i) {
   Console.WriteLine("Printing int: {0}", i );
}
void print(double f) {
   Console.WriteLine("Printing float: {0}" , f);
}
void print(string s) {
   Console.WriteLine("Printing string: {0}", s);
}
static void Main(string[] args) {
   Printdata p = new Printdata();
// Call print to print integer
p.print(5);
// Call print to print float
p.print(500.263);
         // Call print to print string
         p.print("Hello C++");
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Printing int: 5 Printing float: 500.263 Printing string: Hello C++

=== Dynamic Polymorphism

C# allows you to create abstract classes that are used to provide partial class implementation of an interface. Implementation is completed when a derived class inherits from it. *Abstract* classes contain abstract methods, which are implemented by the derived class. The derived classes have more specialized functionality.

Here are the rules about abstract classes −

. You cannot create an instance of an abstract class


. You cannot declare an abstract method outside an abstract class


. When a class is declared *sealed*, it cannot be inherited, abstract classes cannot be declared sealed.


The following program demonstrates an abstract class −

[source]

using System;

namespace PolymorphismApplication {

abstract class Shape {
   public abstract int area();
}
class Rectangle:  Shape {
   private int length;
   private int width;
public Rectangle( int a = 0, int b = 0) {
   length = a;
   width = b;
}
   public override int area () {
      Console.WriteLine("Rectangle class area :");
      return (width * length);
   }
}
class RectangleTester {
      static void Main(string[] args) {
         Rectangle r = new Rectangle(10, 7);
         double a = r.area();
         Console.WriteLine("Area: {0}",a);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Rectangle class area : Area: 70

When you have a function defined in a class that you want to be implemented in an inherited class(es), you use *virtual* functions. The virtual functions could be implemented differently in different inherited class and the call to these functions will be decided at runtime.

Dynamic polymorphism is implemented by *abstract classes* and *virtual functions*.

The following program demonstrates this −

[source]

using System;

namespace PolymorphismApplication {

class Shape {
   protected int width, height;
public Shape( int a = 0, int b = 0) {
   width = a;
   height = b;
}
   public virtual int area() {
      Console.WriteLine("Parent class area :");
      return 0;
   }
}
class Rectangle: Shape {
   public Rectangle( int a = 0, int b = 0): base(a, b) {
}
   public override int area () {
      Console.WriteLine("Rectangle class area :");
      return (width * height);
   }
}
class Triangle: Shape {
   public Triangle(int a = 0, int b = 0): base(a, b) {
}
   public override int area() {
      Console.WriteLine("Triangle class area :");
      return (width * height / 2);
   }
}
class Caller {
   public void CallArea(Shape sh) {
      int a;
      a = sh.area();
      Console.WriteLine("Area: {0}", a);
   }
}
class Tester {
      static void Main(string[] args) {
         Caller c = new Caller();
         Rectangle r = new Rectangle(10, 7);
         Triangle t = new Triangle(10, 5);
         c.CallArea(r);
         c.CallArea(t);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Rectangle class area: Area: 70 Triangle class area: Area: 25

== C

You can redefine or overload most of the built-in operators available in C#. Thus a programmer can use operators with user-defined types as well. Overloaded operators are functions with special names the keyword *operator* followed by the symbol for the operator being defined. similar to any other function, an overloaded operator has a return type and a parameter list.

For example, go through the following function −

[source]

public static Box operator+ (Box b, Box c) { Box box = new Box(); box.length = b.length + c.length; box.breadth = b.breadth + c.breadth; box.height = b.height + c.height; return box; }

The above function implements the addition operator (+) for a user-defined class Box. It adds the attributes of two Box objects and returns the resultant Box object.


=== Implementing the Operator Overloading

The following program shows the complete implementation −

[source]

using System;

namespace OperatorOvlApplication {

class Box {
   private double length;   // Length of a box
   private double breadth;  // Breadth of a box
   private double height;   // Height of a box
public double getVolume() {
   return length * breadth * height;
}
public void setLength( double len ) {
   length = len;
}
public void setBreadth( double bre ) {
   breadth = bre;
}
public void setHeight( double hei ) {
   height = hei;
}
   // Overload + operator to add two Box objects.
   public static Box operator+ (Box b, Box c) {
      Box box = new Box();
      box.length = b.length + c.length;
      box.breadth = b.breadth + c.breadth;
      box.height = b.height + c.height;
      return box;
   }
}
class Tester {
static void Main(string[] args) {
   Box Box1 = new Box();   // Declare Box1 of type Box
   Box Box2 = new Box();   // Declare Box2 of type Box
   Box Box3 = new Box();   // Declare Box3 of type Box
   double volume = 0.0;    // Store the volume of a box here
// box 1 specification
Box1.setLength(6.0);
Box1.setBreadth(7.0);
Box1.setHeight(5.0);
// box 2 specification
Box2.setLength(12.0);
Box2.setBreadth(13.0);
Box2.setHeight(10.0);
// volume of box 1
volume = Box1.getVolume();
Console.WriteLine("Volume of Box1 : {0}", volume);
// volume of box 2
volume = Box2.getVolume();
Console.WriteLine("Volume of Box2 : {0}", volume);
// Add two object as follows:
Box3 = Box1 + Box2;
         // volume of box 3
         volume = Box3.getVolume();
         Console.WriteLine("Volume of Box3 : {0}", volume);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Volume of Box1 : 210 Volume of Box2 : 1560 Volume of Box3 : 5400

=== Overloadable and Non-Overloadable Operators

The following table describes the overload ability of the operators in C# −
[%autowidth]
|===

|Sr.No.|Operators & Description
|1|*+, -, !, ~, ++, --*
These unary operators take one operand and can be overloaded.

|2|*+, -, *, /, %*
These binary operators take one operand and can be overloaded.

|3|*==, !=, <, >, <=, >=*
The comparison operators can be overloaded.

|4|*&&, ||*
The conditional logical operators cannot be overloaded directly.

|5|*+=, -=, *=, /=, %=*
The assignment operators cannot be overloaded.

|6|*=, ., ?:, ->, new, is, sizeof, typeof*
These operators cannot be overloaded.

|===


=== Example

In the light of the above discussions, let us extend the preceding example, and overload few more operators −

[source]

using System;

namespace OperatorOvlApplication {

class Box {
   private double length;    // Length of a box
   private double breadth;   // Breadth of a box
   private double height;    // Height of a box
public double getVolume() {
   return length * breadth * height;
}
public void setLength( double len ) {
   length = len;
}
public void setBreadth( double bre ) {
   breadth = bre;
}
public void setHeight( double hei ) {
   height = hei;
}
// Overload + operator to add two Box objects.
public static Box operator+ (Box b, Box c) {
   Box box = new Box();
   box.length = b.length + c.length;
   box.breadth = b.breadth + c.breadth;
   box.height = b.height + c.height;
   return box;
}
public static bool operator == (Box lhs, Box rhs) {
   bool status = false;
   if (lhs.length == rhs.length && lhs.height == rhs.height && lhs.breadth == rhs.breadth) {
      status = true;
   }
   return status;
}
public static bool operator !=(Box lhs, Box rhs) {
   bool status = false;
   if (lhs.length != rhs.length || lhs.height != rhs.height || lhs.breadth != rhs.breadth) {
      status = true;
   }
   return status;
}
public static bool operator <(Box lhs, Box rhs) {
   bool status = false;
   if (lhs.length < rhs.length && lhs.height < rhs.height && lhs.breadth < rhs.breadth) {
      status = true;
   }
   return status;
}
public static bool operator >(Box lhs, Box rhs) {
   bool status = false;
   if (lhs.length > rhs.length && lhs.height > rhs.height && lhs.breadth > rhs.breadth) {
      status = true;
   }
   return status;
}
public static bool operator <=(Box lhs, Box rhs) {
   bool status = false;
   if (lhs.length <= rhs.length && lhs.height <= rhs.height && lhs.breadth <= rhs.breadth) {
      status = true;
   }
   return status;
}
public static bool operator >=(Box lhs, Box rhs) {
   bool status = false;
   if (lhs.length >= rhs.length && lhs.height >= rhs.height && lhs.breadth >= rhs.breadth) {
      status = true;
   }
   return status;
}
   public override string ToString() {
      return String.Format("({0}, {1}, {2})", length, breadth, height);
   }
}
class Tester {
static void Main(string[] args) {
   Box Box1 = new Box();   // Declare Box1 of type Box
   Box Box2 = new Box();   // Declare Box2 of type Box
   Box Box3 = new Box();   // Declare Box3 of type Box
   Box Box4 = new Box();
   double volume = 0.0;    // Store the volume of a box here
// box 1 specification
Box1.setLength(6.0);
Box1.setBreadth(7.0);
Box1.setHeight(5.0);
// box 2 specification
Box2.setLength(12.0);
Box2.setBreadth(13.0);
Box2.setHeight(10.0);
//displaying the Boxes using the overloaded ToString():
Console.WriteLine("Box 1: {0}", Box1.ToString());
Console.WriteLine("Box 2: {0}", Box2.ToString());
// volume of box 1
volume = Box1.getVolume();
Console.WriteLine("Volume of Box1 : {0}", volume);
// volume of box 2
volume = Box2.getVolume();
Console.WriteLine("Volume of Box2 : {0}", volume);
// Add two object as follows:
Box3 = Box1 + Box2;
Console.WriteLine("Box 3: {0}", Box3.ToString());
// volume of box 3
volume = Box3.getVolume();
Console.WriteLine("Volume of Box3 : {0}", volume);
//comparing the boxes
if (Box1 > Box2)
   Console.WriteLine("Box1 is greater than Box2");
else
   Console.WriteLine("Box1 is  greater than Box2");
if (Box1 < Box2)
   Console.WriteLine("Box1 is less than Box2");
else
   Console.WriteLine("Box1 is not less than Box2");
if (Box1 >= Box2)
   Console.WriteLine("Box1 is greater or equal to Box2");
else
   Console.WriteLine("Box1 is not greater or equal to Box2");
if (Box1 <= Box2)
   Console.WriteLine("Box1 is less or equal to Box2");
else
   Console.WriteLine("Box1 is not less or equal to Box2");
if (Box1 != Box2)
   Console.WriteLine("Box1 is not equal to Box2");
else
   Console.WriteLine("Box1 is not greater or equal to Box2");
Box4 = Box3;
if (Box3 == Box4)
   Console.WriteLine("Box3 is equal to Box4");
else
   Console.WriteLine("Box3 is not equal to Box4");
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Box 1: (6, 7, 5) Box 2: (12, 13, 10) Volume of Box1 : 210 Volume of Box2 : 1560 Box 3: (18, 20, 15) Volume of Box3 : 5400 Box1 is not greater than Box2 Box1 is less than Box2 Box1 is not greater or equal to Box2 Box1 is less or equal to Box2 Box1 is not equal to Box2 Box3 is equal to Box4

== C

An interface is defined as a syntactical contract that all the classes inheriting the interface should follow. The interface defines the *'what'* part of the syntactical contract and the deriving classes define the *'how'* part of the syntactical contract.

Interfaces define properties, methods, and events, which are the members of the interface. Interfaces contain only the declaration of the members. It is the responsibility of the deriving class to define the members. It often helps in providing a standard structure that the deriving classes would follow.

Abstract classes to some extent serve the same purpose, however, they are mostly used when only few methods are to be declared by the base class and the deriving class implements the functionalities.


=== Declaring Interfaces

Interfaces are declared using the interface keyword. It is similar to class declaration. Interface statements are public by default. Following is an example of an interface declaration −

[source]

public interface ITransactions { // interface members void showTransaction(); double getAmount(); }

=== Example

The following example demonstrates implementation of the above interface −

[source]

using System.Collections.Generic; using System.Linq; using System.Text; using System;

namespace InterfaceApplication {

public interface ITransactions {
   // interface members
   void showTransaction();
   double getAmount();
}
public class Transaction : ITransactions {
   private string tCode;
   private string date;
   private double amount;
public Transaction() {
   tCode = " ";
   date = " ";
   amount = 0.0;
}
public Transaction(string c, string d, double a) {
   tCode = c;
   date = d;
   amount = a;
}
public double getAmount() {
   return amount;
}
   public void showTransaction() {
      Console.WriteLine("Transaction: {0}", tCode);
      Console.WriteLine("Date: {0}", date);
      Console.WriteLine("Amount: {0}", getAmount());
   }
}
class Tester {
      static void Main(string[] args) {
         Transaction t1 = new Transaction("001", "8/10/2012", 78900.00);
         Transaction t2 = new Transaction("002", "9/10/2012", 451900.00);
         t1.showTransaction();
         t2.showTransaction();
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Transaction: 001 Date: 8/10/2012 Amount: 78900 Transaction: 002 Date: 9/10/2012 Amount: 451900

== C

A *namespace* is designed for providing a way to keep one set of names separate from another. The class names declared in one namespace does not conflict with the same class names declared in another.


=== Defining a Namespace

A namespace definition begins with the keyword *namespace* followed by the namespace name as follows −

[source]

namespace namespace_name { // code declarations }

To call the namespace-enabled version of either function or variable, prepend the namespace name as follows −

[source]

namespace_name.item_name;

The following program demonstrates use of namespaces −

[source]

using System;

namespace first_space {

class namespace_cl {
      public void func() {
         Console.WriteLine("Inside first_space");
      }
   }
}

namespace second_space {

class namespace_cl {
      public void func() {
         Console.WriteLine("Inside second_space");
      }
   }
}

class TestClass {

   static void Main(string[] args) {
      first_space.namespace_cl fc = new first_space.namespace_cl();
      second_space.namespace_cl sc = new second_space.namespace_cl();
      fc.func();
      sc.func();
      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Inside first_space Inside second_space

=== The using Keyword

The *using* keyword states that the program is using the names in the given namespace. For example, we are using the *System* namespace in our programs. The class Console is defined there. We just write −

[source]

Console.WriteLine ("Hello there");

We could have written the fully qualified name as −

[source]

System.Console.WriteLine("Hello there");

You can also avoid prepending of namespaces with the *using* namespace directive. This directive tells the compiler that the subsequent code is making use of names in the specified namespace. The namespace is thus implied for the following code −

Let us rewrite our preceding example, with using directive −

[source]

using System; using first_space; using second_space;

namespace first_space {

class abc {
      public void func() {
         Console.WriteLine("Inside first_space");
      }
   }
}

namespace second_space {

class efg {
      public void func() {
         Console.WriteLine("Inside second_space");
      }
   }
}

class TestClass {

   static void Main(string[] args) {
      abc fc = new abc();
      efg sc = new efg();
      fc.func();
      sc.func();
      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Inside first_space Inside second_space

=== Nested Namespaces

You can define one namespace inside another namespace as follows −

[source]

namespace namespace_name1 {

   // code declarations
   namespace namespace_name2 {
      // code declarations
   }
}
You can access members of nested namespace by using the dot (.) operator as follows −

[source]

using System; using first_space; using first_space.second_space;

namespace first_space {

class abc {
   public void func() {
      Console.WriteLine("Inside first_space");
   }
}
namespace second_space {
class efg {
         public void func() {
            Console.WriteLine("Inside second_space");
         }
      }
   }
}

class TestClass {

   static void Main(string[] args) {
      abc fc = new abc();
      efg sc = new efg();
      fc.func();
      sc.func();
      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Inside first_space Inside second_space

== C

The preprocessor directives give instruction to the compiler to preprocess the information before actual compilation starts.

All preprocessor directives begin with #, and only white-space characters may appear before a preprocessor directive on a line. Preprocessor directives are not statements, so they do not end with a semicolon (;).

C# compiler does not have a separate preprocessor; however, the directives are processed as if there was one. In C# the preprocessor directives are used to help in conditional compilation. Unlike C and C++ directives, they are not used to create macros. A preprocessor directive must be the only instruction on a line.


=== Preprocessor Directives in C

The following table lists the preprocessor directives available in C# −
[%autowidth]
|===

|Sr.No.|Preprocessor Directive & Description
|1|*#define*
It defines a sequence of characters, called symbol.

|2|*#undef*
It allows you to undefine a symbol.

|3|*#if*
It allows testing a symbol or symbols to see if they evaluate to true.

|4|*#else*
It allows to create a compound conditional directive, along with #if.

|5|*#elif*
It allows creating a compound conditional directive.

|6|*#endif*
Specifies the end of a conditional directive.

|7|*#line*
It lets you modify the compiler's line number and (optionally) the file name output for errors and warnings.

|8|*#error*
It allows generating an error from a specific location in your code.

|9|*#warning*
It allows generating a level one warning from a specific location in your code.

|10|*#region*
It lets you specify a block of code that you can expand or collapse when using the outlining feature of the Visual Studio Code Editor.

|11|*#endregion*
It marks the end of a #region block.

|===


=== The

The #define preprocessor directive creates symbolic constants.

#define lets you define a symbol such that, by using the symbol as the expression passed to the #if directive, the expression evaluates to true. Its syntax is as follows −

[source]

#define symbol

The following program illustrates this −

[source]

#define PI using System;

namespace PreprocessorDAppl {

class Program {
      static void Main(string[] args) {
         #if (PI)
            Console.WriteLine("PI is defined");
         #else
            Console.WriteLine("PI is not defined");
         #endif
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

PI is defined

=== Conditional Directives

You can use the #if directive to create a conditional directive. Conditional directives are useful for testing a symbol or symbols to check if they evaluate to true. If they do evaluate to true, the compiler evaluates all the code between the #if and the next directive.

Syntax for conditional directive is −

[source]

#if symbol [operator symbol]…​

Where, symbol is the name of the symbol you want to test. You can also use true and false or prepend the symbol with the negation operator.

The operator symbol is the operator used for evaluating the symbol. Operators could be either of the following −

. == (equality)

. != (inequality)

. && (and)

. || (or)

You can also group symbols and operators with parentheses. Conditional directives are used for compiling code for a debug build or when compiling for a specific configuration. A conditional directive beginning with a *#if* directive must explicitly be terminated with a *#endif* directive.

The following program demonstrates use of conditional directives −

[source]

#define DEBUG #define VC_V10 using System;

public class TestClass {

   public static void Main() {
      #if (DEBUG && !VC_V10)
         Console.WriteLine("DEBUG is defined");
      #elif (!DEBUG && VC_V10)
         Console.WriteLine("VC_V10 is defined");
      #elif (DEBUG && VC_V10)
         Console.WriteLine("DEBUG and VC_V10 are defined");
      #else
         Console.WriteLine("DEBUG and VC_V10 are not defined");
      #endif
      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

DEBUG and VC_V10 are defined

== C

A *regular expression* is a pattern that could be matched against an input text. The .Net framework provides a regular expression engine that allows such matching. A pattern consists of one or more character literals, operators, or constructs.


=== Constructs for Defining Regular Expressions

There are various categories of characters, operators, and constructs that lets you to define regular expressions. Click the following links to find these constructs.

. link:../csharp/csharp_character_escapes.html[Character escapes]


. link:../csharp/csharp_character_classes.html[Character classes]


. link:../csharp/csharp_anchors.html[Anchors]


. link:../csharp/csharp_grouping_constructs.html[Grouping constructs]


. link:../csharp/csharp_quantifiers.html[Quantifiers]


. link:../csharp/csharp_backreference_constructs.html[Backreference constructs]


. link:../csharp/csharp_alternation_constructs.html[Alternation constructs]


. link:../csharp/csharp_substitutions.html[Substitutions]


. link:../csharp/csharp_miscellaneous_constructs.html[Miscellaneous constructs]



=== The Regex Class

The Regex class is used for representing a regular expression. It has the following commonly used methods −
[%autowidth]
|===

|Sr.No.|Methods & Description
|1|*public bool IsMatch(string input)*
Indicates whether the regular expression specified in the Regex constructor finds a match in a specified input string.

|2|*public bool IsMatch(string input, int startat)*
Indicates whether the regular expression specified in the Regex constructor finds a match in the specified input string, beginning at the specified starting position in the string.

|3|*public static bool IsMatch(string input, string pattern)*
Indicates whether the specified regular expression finds a match in the specified input string.

|4|*public MatchCollection Matches(string input)*
Searches the specified input string for all occurrences of a regular expression.

|5|*public string Replace(string input, string replacement)*
In a specified input string, replaces all strings that match a regular expression pattern with a specified replacement string.

|6|*public string[] Split(string input)*
Splits an input string into an array of substrings at the positions defined by a regular expression pattern specified in the Regex constructor.

|===

For the complete list of methods and properties, please read the Microsoft documentation on C#.


=== Example 1

The following example matches words that start with 'S' −

[source]

using System; using System.Text.RegularExpressions;

namespace RegExApplication {

class Program {
private static void showMatch(string text, string expr) {
   Console.WriteLine("The Expression: " + expr);
   MatchCollection mc = Regex.Matches(text, expr);
   foreach (Match m in mc) {
      Console.WriteLine(m);
   }
}
static void Main(string[] args) {
   string str = "A Thousand Splendid Suns";
         Console.WriteLine("Matching words that start with 'S': ");
         showMatch(str, @"\bS\S*");
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Matching words that start with 'S': The Expression: \bS\S* Splendid Suns

=== Example 2

The following example matches words that start with 'm' and ends with 'e' −

[source]

using System; using System.Text.RegularExpressions;

namespace RegExApplication {

class Program {
   private static void showMatch(string text, string expr) {
      Console.WriteLine("The Expression: " + expr);
      MatchCollection mc = Regex.Matches(text, expr);
      foreach (Match m in mc) {
         Console.WriteLine(m);
      }
   }
   static void Main(string[] args) {
      string str = "make maze and manage to measure it";
         Console.WriteLine("Matching words start with 'm' and ends with 'e':");
         showMatch(str, @"\bm\S*e\b");
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Matching words start with 'm' and ends with 'e': The Expression: \bm\S*e\b make maze manage measure

=== Example 3

This example replaces extra white space −

[source]

using System; using System.Text.RegularExpressions;

namespace RegExApplication {

class Program {
static void Main(string[] args) {
   string input = "Hello   World   ";
   string pattern = "\\s+";
   string replacement = " ";
   Regex rgx = new Regex(pattern);
   string result = rgx.Replace(input, replacement);
         Console.WriteLine("Original String: {0}", input);
         Console.WriteLine("Replacement String: {0}", result);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Original String: Hello World Replacement String: Hello World

== C

An exception is a problem that arises during the execution of a program. A C# exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero.

Exceptions provide a way to transfer control from one part of a program to another. C# exception handling is built upon four keywords: *try*, *catch*, *finally*, and *throw*.

. *try* − A try block identifies a block of code for which particular exceptions is activated. It is followed by one or more catch blocks.


. *catch* − A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The catch keyword indicates the catching of an exception.


. *finally* − The finally block is used to execute a given set of statements, whether an exception is thrown or not thrown. For example, if you open a file, it must be closed whether an exception is raised or not.


. *throw* − A program throws an exception when a problem shows up. This is done using a throw keyword.



=== Syntax

Assuming a block raises an exception, a method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following −

[source]

try { // statements causing exception } catch( ExceptionName e1 ) { // error handling code } catch( ExceptionName e2 ) { // error handling code } catch( ExceptionName eN ) { // error handling code } finally { // statements to be executed }

You can list down multiple catch statements to catch different type of exceptions in case your try block raises more than one exception in different situations.


=== Exception Classes in C

C# exceptions are represented by classes. The exception classes in C# are mainly directly or indirectly derived from the *System.Exception* class. Some of the exception classes derived from the System.Exception class are the *System.ApplicationException* and * System.SystemException* classes.

The *System.ApplicationException* class supports exceptions generated by application programs. Hence the exceptions defined by the programmers should derive from this class.

The *System.SystemException* class is the base class for all predefined system exception.

The following table provides some of the predefined exception classes derived from the Sytem.SystemException class −
[%autowidth]
|===

|Sr.No.|Exception Class & Description
|1|*System.IO.IOException*
Handles I/O errors.

|2|*System.IndexOutOfRangeException*
Handles errors generated when a method refers to an array index out of range.

|3|*System.ArrayTypeMismatchException*
Handles errors generated when type is mismatched with the array type.

|4|*System.NullReferenceException*
Handles errors generated from referencing a null object.

|5|*System.DivideByZeroException*
Handles errors generated from dividing a dividend with zero.

|6|*System.InvalidCastException*
Handles errors generated during typecasting.

|7|*System.OutOfMemoryException*
Handles errors generated from insufficient free memory.

|8|*System.StackOverflowException*
Handles errors generated from stack overflow.

|===


=== Handling Exceptions

C# provides a structured solution to the exception handling in the form of try and catch blocks. Using these blocks the core program statements are separated from the error-handling statements.

These error handling blocks are implemented using the *try*, *catch*, and *finally* keywords. Following is an example of throwing an exception when dividing by zero condition occurs −

[source]

using System;

namespace ErrorHandlingApplication {

class DivNumbers {
   int result;
DivNumbers() {
   result = 0;
}
public void division(int num1, int num2) {
   try {
      result = num1 / num2;
   } catch (DivideByZeroException e) {
      Console.WriteLine("Exception caught: {0}", e);
   } finally {
      Console.WriteLine("Result: {0}", result);
   }
}
      static void Main(string[] args) {
         DivNumbers d = new DivNumbers();
         d.division(25, 0);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Exception caught: System.DivideByZeroException: Attempted to divide by zero. at …​ Result: 0

=== Creating User-Defined Exceptions

You can also define your own exception. User-defined exception classes are derived from the *Exception* class. The following example demonstrates this −

[source]

using System;

namespace UserDefinedException {

class TestTemperature {
      static void Main(string[] args) {
         Temperature temp = new Temperature();
         try {
            temp.showTemp();
         } catch(TempIsZeroException e) {
            Console.WriteLine("TempIsZeroException: {0}", e.Message);
         }
         Console.ReadKey();
      }
   }
}

public class TempIsZeroException: Exception {

public TempIsZeroException(string message): base(message) {
   }
}

public class Temperature { int temperature = 0;

public void showTemp() {
      if(temperature == 0) {
         throw (new TempIsZeroException("Zero Temperature found"));
      } else {
         Console.WriteLine("Temperature: {0}", temperature);
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

TempIsZeroException: Zero Temperature found

=== Throwing Objects

You can throw an object if it is either directly or indirectly derived from the *System.Exception* class. You can use a throw statement in the catch block to throw the present object as −

[source]

Catch(Exception e) { …​ Throw e }

== C

A *file* is a collection of data stored in a disk with a specific name and a directory path. When a file is opened for reading or writing, it becomes a *stream*.

The stream is basically the sequence of bytes passing through the communication path. There are two main streams: the *input stream* and the *output stream*. The* input stream* is used for reading data from file (read operation) and the *output stream* is used for writing into the file (write operation).


=== C

The System.IO namespace has various classes that are used for performing numerous operations with files, such as creating and deleting files, reading from or writing to a file, closing a file etc.

The following table shows some commonly used non-abstract classes in the System.IO namespace −
[%autowidth]
|===

|Sr.No.|I/O Class & Description
|1|*BinaryReader*
Reads primitive data from a binary stream.

|2|*BinaryWriter*
Writes primitive data in binary format.

|3|*BufferedStream*
A temporary storage for a stream of bytes.

|4|*Directory*
Helps in manipulating a directory structure.

|5|*DirectoryInfo*
Used for performing operations on directories.

|6|*DriveInfo*
Provides information for the drives.

|7|*File*
Helps in manipulating files.

|8|*FileInfo*
Used for performing operations on files.

|9|*FileStream*
Used to read from and write to any location in a file.

|10|*MemoryStream*
Used for random access to streamed data stored in memory.

|11|*Path*
Performs operations on path information.

|12|*StreamReader*
Used for reading characters from a byte stream.

|13|*StreamWriter*
Is used for writing characters to a stream.

|14|*StringReader*
Is used for reading from a string buffer.

|15|*StringWriter*
Is used for writing into a string buffer.

|===


=== The FileStream Class

The *FileStream* class in the System.IO namespace helps in reading from, writing to and closing files. This class derives from the abstract class Stream.

You need to create a *FileStream* object to create a new file or open an existing file. The syntax for creating a *FileStream* object is as follows −

[source]

FileStream <object_name> = new FileStream( <file_name>, <FileMode Enumerator>, <FileAccess Enumerator>, <FileShare Enumerator>);

For example, we create a FileStream object *F* for reading a file named *sample.txt as shown* −

[source]

FileStream F = new FileStream("sample.txt", FileMode.Open, FileAccess.Read, FileShare.Read);

[%autowidth]
|===

|Sr.No.|Parameter & Description
|1|*FileMode*
The *FileMode* enumerator defines various methods for opening files. The members of the FileMode enumerator are −
*Append* − It opens an existing file and puts cursor at the end of file, or creates the file, if the file does not exist.
*Create* − It creates a new file.
*CreateNew* − It specifies to the operating system, that it should create a new file.
*Open* − It opens an existing file.
*OpenOrCreate* − It specifies to the operating system that it should open a file if it exists, otherwise it should create a new file.
*Truncate* − It opens an existing file and truncates its size to zero bytes.

|2|*FileAccess*
*FileAccess* enumerators have members: *Read*, *ReadWrite* and *Write*.

|3|*FileShare*
*FileShare* enumerators have the following members −
*Inheritable* − It allows a file handle to pass inheritance to the child processes
*None* − It declines sharing of the current file
*Read* − It allows opening the file for readin.
*ReadWrite* − It allows opening the file for reading and writing
*Write* − It allows opening the file for writing

|===


=== Example

The following program demonstrates use of the *FileStream *class −

[source]

using System; using System.IO;

namespace FileIOApplication {

class Program {
static void Main(string[] args) {
   FileStream F = new FileStream("test.dat", FileMode.OpenOrCreate,
      FileAccess.ReadWrite);
for (int i = 1; i <= 20; i++) {
   F.WriteByte((byte)i);
}
         F.Position = 0;
         for (int i = 0; i <= 20; i++) {
            Console.Write(F.ReadByte() + " ");
         }
         F.Close();
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 -1

=== Advanced File Operations in C

The preceding example provides simple file operations in C#. However, to utilize the immense powers of C# System.IO classes, you need to know the commonly used properties and methods of these classes.
[%autowidth]
|===

|Sr.No.|Topic & Description
|1|link:../csharp/csharp_text_files.html[Reading from and Writing into Text files]It involves reading from and writing into text files. The *StreamReader* and *StreamWriter* class helps to accomplish it.

|2|link:../csharp/csharp_binary_files.html[Reading from and Writing into Binary files]It involves reading from and writing into binary files. The *BinaryReader* and *BinaryWriter* class helps to accomplish this.

|3|link:../csharp/csharp_windows_file_system.html[Manipulating the Windows file system]It gives a C# programamer the ability to browse and locate Windows files and directories.

|===


== C

An *attribute* is a declarative tag that is used to convey information to runtime about the behaviors of various elements like classes, methods, structures, enumerators, assemblies etc. in your program. You can add declarative information to a program by using an attribute. A declarative tag is depicted by square ([ ]) brackets placed above the element it is used for.

Attributes are used for adding metadata, such as compiler instruction and other information such as comments, description, methods and classes to a program. The .Net Framework provides two types of attributes: the pre-defined attributes and custom built attributes.


=== Specifying an Attribute

Syntax for specifying an attribute is as follows −

[source]

element

Name of the attribute and its values are specified within the square brackets, before the element to which the attribute is applied. Positional parameters specify the essential information and the name parameters specify the optional information.


=== Predefined Attributes

The .Net Framework provides three pre-defined attributes −

. AttributeUsage

. Conditional

. Obsolete


=== AttributeUsage

The pre-defined attribute *AttributeUsage* describes how a custom attribute class can be used. It specifies the types of items to which the attribute can be applied.

Syntax for specifying this attribute is as follows −

[source]

[AttributeUsage ( validon, AllowMultiple = allowmultiple, Inherited = inherited )]

Where,

. The parameter validon specifies the language elements on which the attribute can be placed. It is a combination of the value of an enumerator AttributeTargets. The default value is  AttributeTargets.All.


. The parameter allowmultiple (optional) provides value for the AllowMultiple property of this attribute, a Boolean value. If this is true, the attribute is multiuse. The default is false (single-use).


. The parameter inherited (optional) provides value for the Inherited property of this attribute, a Boolean value. If it is true, the attribute is inherited by derived classes. The default value is false (not inherited).


For example,

[source]

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Field | AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = true)]

=== Conditional

This predefined attribute marks a conditional method whose execution depends on a specified preprocessing identifier.

It causes conditional compilation of method calls, depending on the specified value such as *Debug* or * Trace*. For example, it displays the values of the variables while debugging a code.

Syntax for specifying this attribute is as follows −

[source]

[Conditional( conditionalSymbol )]

For example,

[source]
The following example demonstrates the attribute −

[source]

#define DEBUG using System; using System.Diagnostics;

public class Myclass { [Conditional("DEBUG")]

   public static void Message(string msg) {
      Console.WriteLine(msg);
   }
}

class Test { static void function1() { Myclass.Message("In Function 1."); function2(); }

static void function2() {
   Myclass.Message("In Function 2.");
}
   public static void Main() {
      Myclass.Message("In Main function.");
      function1();
      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

In Main function In Function 1 In Function 2

=== Obsolete

This predefined attribute marks a program entity that should not be used. It enables you to inform the compiler to discard a particular target element. For example, when a new method is being used in a class and if you still want to retain the old method in the class, you may mark it as obsolete by displaying a message the new method should be used, instead of the old method.

Syntax for specifying this attribute is as follows −

[source]

[Obsolete ( message )]

[Obsolete ( message, iserror )]

Where,

. The parameter message, is a string describing the reason why the item is obsolete and what to use instead.


. The parameter iserror, is a Boolean value. If its value is true, the compiler should treat the use of the item as an error. Default value is false (compiler generates a warning).


The following program demonstrates this −

[source]

using System;

public class MyClass { [Obsolete("Don’t use OldMethod, use NewMethod instead", true)]

static void OldMethod() {
   Console.WriteLine("It is the old method");
}
static void NewMethod() {
   Console.WriteLine("It is the new method");
}
   public static void Main() {
      OldMethod();
   }
}
When you try to compile the program, the compiler gives an error message stating −

[source]
Don't use OldMethod, use NewMethod instead
=== Creating Custom Attributes

The .Net Framework allows creation of custom attributes that can be used to store declarative information and can be retrieved at run-time. This information can be related to any target element depending upon the design criteria and application need.

Creating and using custom attributes involve four steps −

. Declaring a custom attribute

. Constructing the custom attribute

. Apply the custom attribute on a target program element

. Accessing Attributes Through Reflection

The Last step involves writing a simple program to read through the metadata to find various notations. Metadata is data about data or information used for describing other data. This program should use reflections for accessing attributes at runtime. This we will discuss in the next chapter.


=== Declaring a Custom Attribute

A new custom attribute should is derived from the *System.Attribute* class. For example,

[source]

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Field | AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = true)]

public class DeBugInfo : System.Attribute

In the preceding code, we have declared a custom attribute named DeBugInfo.


=== Constructing the Custom Attribute

Let us construct a custom attribute named DeBugInfo, which stores the information obtained by debugging any program. Let it store the following information −

. The code number for the bug

. Name of the developer who identified the bug

. Date of last review of the code

. A string message for storing the developer's remarks

The DeBugInfo class has three private properties for storing the first three information and a public property for storing the message. Hence the bug number, developer's name, and date of review are the positional parameters of the DeBugInfo class and the message is an optional or named parameter.

Each attribute must have at least one constructor. The positional parameters should be passed through the constructor. The following code shows the DeBugInfo class −

[source]

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Field | AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = true)]

public class DeBugInfo : System.Attribute { private int bugNo; private string developer; private string lastReview; public string message;

public DeBugInfo(int bg, string dev, string d) {
   this.bugNo = bg;
   this.developer = dev;
   this.lastReview = d;
}
public int BugNo {
   get {
      return bugNo;
   }
}
public string Developer {
   get {
      return developer;
   }
}
public string LastReview {
   get {
      return lastReview;
   }
}
   public string Message {
      get {
         return message;
      }
      set {
         message = value;
      }
   }
}
=== Applying the Custom Attribute

The attribute is applied by placing it immediately before its target −

[source]

class Rectangle { //member variables protected double length; protected double width; public Rectangle(double l, double w) { length = l; width = w; } [DeBugInfo(55, "Zara Ali", "19/10/2012", Message = "Return type mismatch")]

public double GetArea() {
   return length * width;
}
[DeBugInfo(56, "Zara Ali", "19/10/2012")]
   public void Display() {
      Console.WriteLine("Length: {0}", length);
      Console.WriteLine("Width: {0}", width);
      Console.WriteLine("Area: {0}", GetArea());
   }
}
In the next chapter, we retrieve attribute information using a Reflection class object.


== C

*Reflection* objects are used for obtaining type information at runtime. The classes that give access to the metadata of a running program are in the *System.Reflection* namespace.

The *System.Reflection* namespace contains classes that allow you to obtain information about the application and to dynamically add types, values, and objects to the application.


=== Applications of Reflection

Reflection has the following applications −

. It allows view attribute information at runtime.


. It allows examining various types in an assembly and instantiate these types.


. It allows late binding to methods and properties


. It allows creating new types at runtime and then performs some tasks using those types.



=== Viewing Metadata

We have mentioned in the preceding chapter that using reflection you can view the attribute information.

The *MemberInfo* object of the *System.Reflection* class needs to be initialized for discovering the attributes associated with a class. To do this, you define an object of the target class, as −

[source]

System.Reflection.MemberInfo info = typeof(MyClass);

The following program demonstrates this −

[source]

using System;

public class HelpAttribute : System.Attribute { public readonly string Url;

public string Topic   // Topic is a named parameter {
   get {
      return topic;
   }
   set {
      topic = value;
   }
}
   public HelpAttribute(string url)   // url is a positional parameter {
      this.Url = url;
   }
   private string topic;
}

class MyClass {

}

namespace AttributeAppl {

class Program {
      static void Main(string[] args) {
         System.Reflection.MemberInfo info = typeof(MyClass);
         object[] attributes = info.GetCustomAttributes(true);
         for (int i = 0; i < attributes.Length; i++) {
            System.Console.WriteLine(attributes[i]);
         }
         Console.ReadKey();
      }
   }
}
When it is compiled and run, it displays the name of the custom attributes attached to the class MyClass −

[source]

HelpAttribute

=== Example

In this example, we use the DeBugInfo attribute created in the previous chapter and use reflection to read metadata in the Rectangle class.

[source]

using System; using System.Reflection;

namespace BugFixApplication { //a custom attribute BugFix to be //assigned to a class and its members [AttributeUsage(AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Field | AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = true)]

public class DeBugInfo : System.Attribute {
   private int bugNo;
   private string developer;
   private string lastReview;
   public string message;
public DeBugInfo(int bg, string dev, string d) {
   this.bugNo = bg;
   this.developer = dev;
   this.lastReview = d;
}
public int BugNo {
   get {
      return bugNo;
   }
}
public string Developer {
   get {
      return developer;
   }
}
public string LastReview {
   get {
      return lastReview;
   }
}
   public string Message {
      get {
         return message;
      }
      set {
         message = value;
      }
   }
}
[DeBugInfo(45, "Zara Ali", "12/8/2012", Message = "Return type mismatch")]
[DeBugInfo(49, "Nuha Ali", "10/10/2012", Message = "Unused variable")]
class Rectangle {
   //member variables
   protected double length;
   protected double width;
   public Rectangle(double l, double w) {
      length = l;
      width = w;
   }
[DeBugInfo(55, "Zara Ali", "19/10/2012", Message = "Return type mismatch")]
public double GetArea() {
   return length * width;
}
   [DeBugInfo(56, "Zara Ali", "19/10/2012")]
   public void Display() {
      Console.WriteLine("Length: {0}", length);
      Console.WriteLine("Width: {0}", width);
      Console.WriteLine("Area: {0}", GetArea());
   }
}//end class Rectangle
class ExecuteRectangle {
   static void Main(string[] args) {
      Rectangle r = new Rectangle(4.5, 7.5);
      r.Display();
      Type type = typeof(Rectangle);
//iterating through the attribtues of the Rectangle class
foreach (Object attributes in type.GetCustomAttributes(false)) {
   DeBugInfo dbi = (DeBugInfo)attributes;
   if (null != dbi) {
      Console.WriteLine("Bug no: {0}", dbi.BugNo);
      Console.WriteLine("Developer: {0}", dbi.Developer);
      Console.WriteLine("Last Reviewed: {0}", dbi.LastReview);
      Console.WriteLine("Remarks: {0}", dbi.Message);
   }
}
//iterating through the method attribtues
foreach (MethodInfo m in type.GetMethods()) {
foreach (Attribute a in m.GetCustomAttributes(true)) {
   DeBugInfo dbi = (DeBugInfo)a;
               if (null != dbi) {
                  Console.WriteLine("Bug no: {0}, for Method: {1}", dbi.BugNo, m.Name);
                  Console.WriteLine("Developer: {0}", dbi.Developer);
                  Console.WriteLine("Last Reviewed: {0}", dbi.LastReview);
                  Console.WriteLine("Remarks: {0}", dbi.Message);
               }
            }
         }
         Console.ReadLine();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Length: 4.5 Width: 7.5 Area: 33.75 Bug No: 49 Developer: Nuha Ali Last Reviewed: 10/10/2012 Remarks: Unused variable Bug No: 45 Developer: Zara Ali Last Reviewed: 12/8/2012 Remarks: Return type mismatch Bug No: 55, for Method: GetArea Developer: Zara Ali Last Reviewed: 19/10/2012 Remarks: Return type mismatch Bug No: 56, for Method: Display Developer: Zara Ali Last Reviewed: 19/10/2012 Remarks:

== C

*Properties* are named members of classes, structures, and interfaces. Member variables or methods in a class or structures are called *Fields*. Properties are an extension of fields and are accessed using the same syntax. They use *accessors* through which the values of the private fields can be read, written or manipulated.

Properties do not name the storage locations. Instead, they have *accessors* that read, write, or compute their values.

For example, let us have a class named Student, with private fields for age, name, and code. We cannot directly access these fields from outside the class scope, but we can have properties for accessing these private fields.


=== Accessors

The *accessor* of a property contains the executable statements that helps in getting (reading or computing) or setting (writing) the property. The accessor declarations can contain a get accessor, a set accessor, or both. For example −

[source]

public string Code { get { return code; } set { code = value; } }

public string Name { get { return name; } set { name = value; } }

public int Age { get { return age; } set { age = value; } }

=== Example

The following example demonstrates use of properties −

[source]

using System; namespace tutorialspoint { class Student { private string code = "N.A"; private string name = "not known"; private int age = 0;

// Declare a Code property of type string:
public string Code {
   get {
      return code;
   }
   set {
      code = value;
   }
}
// Declare a Name property of type string:
public string Name {
   get {
      return name;
   }
   set {
      name = value;
   }
}
   // Declare a Age property of type int:
   public int Age {
      get {
         return age;
      }
      set {
         age = value;
      }
   }
   public override string ToString() {
      return "Code = " + Code +", Name = " + Name + ", Age = " + Age;
   }
}
class ExampleDemo {
   public static void Main() {
// Create a new Student object:
Student s = new Student();
// Setting code, name and the age of the student
s.Code = "001";
s.Name = "Zara";
s.Age = 9;
Console.WriteLine("Student Info: {0}", s);
         //let us increase age
         s.Age += 1;
         Console.WriteLine("Student Info: {0}", s);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Student Info: Code = 001, Name = Zara, Age = 9 Student Info: Code = 001, Name = Zara, Age = 10

=== Abstract Properties

An abstract class may have an abstract property, which should be implemented in the derived class. The following program illustrates this −

[source]

using System; namespace tutorialspoint { public abstract class Person { public abstract string Name { get; set; } public abstract int Age { get; set; } }

class Student : Person {
private string code = "N.A";
private string name = "N.A";
private int age = 0;
// Declare a Code property of type string:
public string Code {
   get {
      return code;
   }
   set {
      code = value;
   }
}
// Declare a Name property of type string:
public override string Name {
   get {
      return name;
   }
   set {
      name = value;
   }
}
   // Declare a Age property of type int:
   public override int Age {
      get {
         return age;
      }
      set {
         age = value;
      }
   }
   public override string ToString() {
      return "Code = " + Code +", Name = " + Name + ", Age = " + Age;
   }
}
class ExampleDemo {
   public static void Main() {
      // Create a new Student object:
      Student s = new Student();
// Setting code, name and the age of the student
s.Code = "001";
s.Name = "Zara";
s.Age = 9;
Console.WriteLine("Student Info:- {0}", s);
         //let us increase age
         s.Age += 1;
         Console.WriteLine("Student Info:- {0}", s);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Student Info: Code = 001, Name = Zara, Age = 9 Student Info: Code = 001, Name = Zara, Age = 10

== C

An *indexer* allows an object to be indexed such as an array. When you define an indexer for a class, this class behaves similar to a *virtual array*. You can then access the instance of this class using the array access operator ([ ]).


=== Syntax

A one dimensional indexer has the following syntax −

[source]

element-type this[int index] {

// The get accessor.
get {
   // return the value specified by index
}
   // The set accessor.
   set {
      // set the value specified by index
   }
}
=== Use of Indexers

Declaration of behavior of an indexer is to some extent similar to a property. similar to the properties, you use * get* and *set* accessors for defining an indexer. However, properties return or set a specific data member, whereas indexers returns or sets a particular value from the object instance. In other words, it breaks the instance data into smaller parts and indexes each part, gets or sets each part.

Defining a property involves providing a property name. Indexers are not defined with names, but with the *this* keyword, which refers to the object instance. The following example demonstrates the concept −

[source]

using System;

namespace IndexerApplication {

class IndexedNames {
   private string[] namelist = new string[size];
   static public int size = 10;
public IndexedNames() {
   for (int i = 0; i < size; i++)
   namelist[i] = "N. A.";
}
public string this[int index] {
get {
   string tmp;
if( index >= 0 && index <= size-1 ) {
   tmp = namelist[index];
} else {
   tmp = "";
}
      return ( tmp );
   }
   set {
      if( index >= 0 && index <= size-1 ) {
         namelist[index] = value;
      }
   }
}
static void Main(string[] args) {
   IndexedNames names = new IndexedNames();
   names[0] = "Zara";
   names[1] = "Riz";
   names[2] = "Nuha";
   names[3] = "Asif";
   names[4] = "Davinder";
   names[5] = "Sunil";
   names[6] = "Rubic";
         for ( int i = 0; i < IndexedNames.size; i++ ) {
            Console.WriteLine(names[i]);
         }
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Zara Riz Nuha Asif Davinder Sunil Rubic N. A. N. A. N. A.

=== Overloaded Indexers

Indexers can be overloaded. Indexers can also be declared with multiple parameters and each parameter may be a different type. It is not necessary that the indexes have to be integers. C# allows indexes to be of other types, for example, a string.

The following example demonstrates overloaded indexers −

[source]

using System;

namespace IndexerApplication {

class IndexedNames {
   private string[] namelist = new string[size];
   static public int size = 10;
public IndexedNames() {
   for (int i = 0; i < size; i++) {
      namelist[i] = "N. A.";
   }
}
public string this[int index] {
get {
   string tmp;
if( index >= 0 && index <= size-1 ) {
   tmp = namelist[index];
} else {
   tmp = "";
}
      return ( tmp );
   }
   set {
      if( index >= 0 && index <= size-1 ) {
         namelist[index] = value;
      }
   }
}
public int this[string name] {
get {
   int index = 0;
   while(index < size) {
      if (namelist[index] == name) {
       return index;
      }
      index++;
   }
   return index;
}
}
static void Main(string[] args) {
   IndexedNames names = new IndexedNames();
   names[0] = "Zara";
   names[1] = "Riz";
   names[2] = "Nuha";
   names[3] = "Asif";
   names[4] = "Davinder";
   names[5] = "Sunil";
   names[6] = "Rubic";
//using the first indexer with int parameter
for (int i = 0; i < IndexedNames.size; i++) {
   Console.WriteLine(names[i]);
}
         //using the second indexer with the string parameter
         Console.WriteLine(names["Nuha"]);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Zara Riz Nuha Asif Davinder Sunil Rubic N. A. N. A. N. A. 2

== C

C# delegates are similar to pointers to functions, in C or C++. A *delegate* is a reference type variable that holds the reference to a method. The reference can be changed at runtime.

Delegates are especially used for implementing events and the call-back methods. All delegates are implicitly derived from the *System.Delegate* class.


=== Declaring Delegates

Delegate declaration determines the methods that can be referenced by the delegate. A delegate can refer to a method, which has the same signature as that of the delegate.

For example, consider a delegate −

[source]

public delegate int MyDelegate (string s);

The preceding delegate can be used to reference any method that has a single string parameter and returns an int type variable.

Syntax for delegate declaration is −

[source]

delegate <return type> <delegate-name> <parameter list>

=== Instantiating Delegates

Once a delegate type is declared, a delegate object must be created with the *new * keyword and be associated with a particular method. When creating a delegate, the argument passed to the *new* expression is written similar to a method call, but without the arguments to the method. For example −

[source]

public delegate void printString(string s); …​ printString ps1 = new printString(WriteToScreen); printString ps2 = new printString(WriteToFile);

Following example demonstrates declaration, instantiation, and use of a delegate that can be used to reference methods that take an integer parameter and returns an integer value.

[source]

using System;

delegate int NumberChanger(int n); namespace DelegateAppl {

class TestDelegate {
   static int num = 10;
public static int AddNum(int p) {
   num += p;
   return num;
}
public static int MultNum(int q) {
   num *= q;
   return num;
}
public static int getNum() {
   return num;
}
static void Main(string[] args) {
   //create delegate instances
   NumberChanger nc1 = new NumberChanger(AddNum);
   NumberChanger nc2 = new NumberChanger(MultNum);
         //calling the methods using the delegate objects
         nc1(25);
         Console.WriteLine("Value of Num: {0}", getNum());
         nc2(5);
         Console.WriteLine("Value of Num: {0}", getNum());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Value of Num: 35 Value of Num: 175

=== Multicasting of a Delegate

Delegate objects can be composed using the "+" operator. A composed delegate calls the two delegates it was composed from. Only delegates of the same type can be composed. The "-" operator can be used to remove a component delegate from a composed delegate.

Using this property of delegates you can create an invocation list of methods that will be called when a delegate is invoked. This is called *multicasting* of a delegate. The following program demonstrates multicasting of a delegate −

[source]

using System;

delegate int NumberChanger(int n); namespace DelegateAppl {

class TestDelegate {
   static int num = 10;
public static int AddNum(int p) {
   num += p;
   return num;
}
public static int MultNum(int q) {
   num *= q;
   return num;
}
public static int getNum() {
   return num;
}
static void Main(string[] args) {
   //create delegate instances
   NumberChanger nc;
   NumberChanger nc1 = new NumberChanger(AddNum);
   NumberChanger nc2 = new NumberChanger(MultNum);
   nc = nc1;
   nc += nc2;
         //calling multicast
         nc(5);
         Console.WriteLine("Value of Num: {0}", getNum());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Value of Num: 75

=== Using Delegates

The following example demonstrates the use of delegate. The delegate printString can be used to reference method that takes a string as input and returns nothing.

We use this delegate to call two methods, the first prints the string to the console, and the second one prints it to a file −

[source]

using System; using System.IO;

namespace DelegateAppl {

class PrintString {
   static FileStream fs;
   static StreamWriter sw;
// delegate declaration
public delegate void printString(string s);
// this method prints to the console
public static void WriteToScreen(string str) {
   Console.WriteLine("The String is: {0}", str);
}
//this method prints to a file
public static void WriteToFile(string s) {
   fs = new FileStream("c:\\message.txt",
   FileMode.Append, FileAccess.Write);
   sw = new StreamWriter(fs);
   sw.WriteLine(s);
   sw.Flush();
   sw.Close();
   fs.Close();
}
// this method takes the delegate as parameter and uses it to
// call the methods as required
public static void sendString(printString ps) {
   ps("Hello World");
}
      static void Main(string[] args) {
         printString ps1 = new printString(WriteToScreen);
         printString ps2 = new printString(WriteToFile);
         sendString(ps1);
         sendString(ps2);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

The String is: Hello World

== C

*Events* are user actions such as key press, clicks, mouse movements, etc., or some occurrence such as system generated notifications. Applications need to respond to events when they occur. For example, interrupts. Events are used for inter-process communication.


=== Using Delegates with Events

The events are declared and raised in a class and associated with the event handlers using delegates within the same class or some other class. The class containing the event is used to publish the event. This is called the *publisher* class. Some other class that accepts this event is called the *subscriber * class. Events use the *publisher-subscriber* model.

A *publisher* is an object that contains the definition of the event and the delegate. The event-delegate association is also defined in this object. A publisher class object invokes the event and it is notified to other objects.

A *subscriber* is an object that accepts the event and provides an event handler. The delegate in the publisher class invokes the method (event handler) of the subscriber class.


=== Declaring Events

To declare an event inside a class, first of all, you must declare a delegate type for the even as:

[source]

public delegate string BoilerLogHandler(string str);

then, declare the event using the *event* keyword −

[source]

event BoilerLogHandler BoilerEventLog;

The preceding code defines a delegate named BoilerLogHandler and an event named BoilerEventLog, which invokes the delegate when it is raised.


=== Example

[source]

using System;

namespace SampleApp { public delegate string MyDel(string str);

class EventProgram {
   event MyDel MyEvent;
public EventProgram() {
   this.MyEvent += new MyDel(this.WelcomeUser);
}
public string WelcomeUser(string username) {
   return "Welcome " + username;
}
static void Main(string[] args) {
   EventProgram obj1 = new EventProgram();
   string result = obj1.MyEvent("Tutorials Point");
   Console.WriteLine(result);
}
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Welcome Tutorials Point

== C

Collection classes are specialized classes for data storage and retrieval. These classes provide support for stacks, queues, lists, and hash tables. Most collection classes implement the same interfaces.

Collection classes serve various purposes, such as allocating memory dynamically to elements and accessing a list of items on the basis of an index etc. These classes create collections of objects of the Object class, which is the base class for all data types in C#.


=== Various Collection Classes and Their Usage

The following are the various commonly used classes of the *System.Collection* namespace. Click the following links to check their detail.
[%autowidth]
|===

|Sr.No.|Class & Description and Useage
|1|link:../csharp/csharp_arraylist.html[ArrayList]It represents ordered collection of an object that can be *indexed* individually.
It is basically an alternative to an array. However, unlike array you can add and remove items from a list at a specified position using an *index* and the array resizes itself automatically. It also allows dynamic memory allocation, adding, searching and sorting items in the list.

|2|link:../csharp/csharp_hashtable.html[Hashtable]It uses a *key* to access the elements in the collection.
A hash table is used when you need to access elements by using key, and you can identify a useful key value. Each item in the hash table has a *key/value* pair. The key is used to access the items in the collection.

|3|link:../csharp/csharp_sortedlist.html[SortedList]It uses a *key* as well as an *index* to access the items in a list.
A sorted list is a combination of an array and a hash table. It contains a list of items that can be accessed using a key or an index. If you access items using an index, it is an ArrayList, and if you access items using a key , it is a Hashtable. The collection of items is always sorted by the key value.

|4|link:../csharp/csharp_stack.html[Stack]It represents a *last-in, first out* collection of object.
It is used when you need a last-in, first-out access of items. When you add an item in the list, it is called *pushing* the item and when you remove it, it is called *popping* the item.

|5|link:../csharp/csharp_queue.html[Queue]It represents a *first-in, first out* collection of object.
It is used when you need a first-in, first-out access of items. When you add an item in the list, it is called *enqueue* and when you remove an item, it is called *deque*.

|6|link:../csharp/csharp_bitarray.html[BitArray]It represents an array of the *binary representation* using the values 1 and 0.
It is used when you need to store the bits but do not know the number of bits in advance. You can access items from the BitArray collection by using an *integer index*, which starts from zero.

|===


== C

*Generics* allow you to define the specification of the data type of programming elements in a class or a method, until it is actually used in the program. In other words, generics allow you to write a class or method that can work with any data type.

You write the specifications for the class or the method, with substitute parameters for data types. When the compiler encounters a constructor for the class or a function call for the method, it generates code to handle the specific data type. A simple example would help understanding the concept −

[source]

using System; using System.Collections.Generic;

namespace GenericApplication {

public class MyGenericArray<T> {
   private T[] array;
public MyGenericArray(int size) {
   array = new T[size + 1];
}
public T getItem(int index) {
   return array[index];
}
   public void setItem(int index, T value) {
      array[index] = value;
   }
}
class Tester {
   static void Main(string[] args) {
//declaring an int array
MyGenericArray<int> intArray = new MyGenericArray<int>(5);
//setting values
for (int c = 0; c < 5; c++) {
   intArray.setItem(c, c*5);
}
//retrieving the values
for (int c = 0; c < 5; c++) {
   Console.Write(intArray.getItem(c) + " ");
}
Console.WriteLine();
//declaring a character array
MyGenericArray<char> charArray = new MyGenericArray<char>(5);
//setting values
for (int c = 0; c < 5; c++) {
   charArray.setItem(c, (char)(c+97));
}
//retrieving the values
for (int c = 0; c< 5; c++) {
   Console.Write(charArray.getItem(c) + " ");
}
Console.WriteLine();
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

0 5 10 15 20 a b c d e

=== Features of Generics

Generics is a technique that enriches your programs in the following ways −

. It helps you to maximize code reuse, type safety, and performance.


. You can create generic collection classes. The .NET Framework class library contains several new generic collection classes in the System.Collections.Generic namespace. You may use these generic collection classes instead of the collection classes in the System.Collections namespace.


. You can create your own generic interfaces, classes, methods, events, and delegates.


. You may create generic classes constrained to enable access to methods on particular data types.


. You may get information on the types used in a generic data type at run-time by means of reflection.



=== Generic Methods

In the previous example, we have used a generic class; we can declare a generic method with a type parameter. The following program illustrates the concept −

[source]

using System; using System.Collections.Generic;

namespace GenericMethodAppl {

class Program {
static void Swap<T>(ref T lhs, ref T rhs) {
   T temp;
   temp = lhs;
   lhs = rhs;
   rhs = temp;
}
static void Main(string[] args) {
   int a, b;
   char c, d;
   a = 10;
   b = 20;
   c = 'I';
   d = 'V';
//display values before swap:
Console.WriteLine("Int values before calling swap:");
Console.WriteLine("a = {0}, b = {1}", a, b);
Console.WriteLine("Char values before calling swap:");
Console.WriteLine("c = {0}, d = {1}", c, d);
//call swap
Swap<int>(ref a, ref b);
Swap<char>(ref c, ref d);
//display values after swap:
Console.WriteLine("Int values after calling swap:");
Console.WriteLine("a = {0}, b = {1}", a, b);
Console.WriteLine("Char values after calling swap:");
Console.WriteLine("c = {0}, d = {1}", c, d);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Int values before calling swap: a = 10, b = 20 Char values before calling swap: c = I, d = V Int values after calling swap: a = 20, b = 10 Char values after calling swap: c = V, d = I

=== Generic Delegates

You can define a generic delegate with type parameters. For example −

[source]

delegate T NumberChanger<T>(T n);

The following example shows use of this delegate −

[source]

using System; using System.Collections.Generic;

delegate T NumberChanger<T>(T n); namespace GenericDelegateAppl {

class TestDelegate {
   static int num = 10;
public static int AddNum(int p) {
   num += p;
   return num;
}
public static int MultNum(int q) {
   num *= q;
   return num;
}
public static int getNum() {
   return num;
}
static void Main(string[] args) {
   //create delegate instances
   NumberChanger<int> nc1 = new NumberChanger<int>(AddNum);
   NumberChanger<int> nc2 = new NumberChanger<int>(MultNum);
         //calling the methods using the delegate objects
         nc1(25);
         Console.WriteLine("Value of Num: {0}", getNum());
         nc2(5);
         Console.WriteLine("Value of Num: {0}", getNum());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Value of Num: 35 Value of Num: 175

== C

We discussed that delegates are used to reference any methods that has the same signature as that of the delegate. In other words, you can call a method that can be referenced by a delegate using that delegate object.

*Anonymous methods* provide a technique to pass a code block as a delegate parameter. Anonymous methods are the methods without a name, just the body.

You need not specify the return type in an anonymous method; it is inferred from the return statement inside the method body.


=== Writing an Anonymous Method

Anonymous methods are declared with the creation of the delegate instance, with a *delegate* keyword. For example,

[source]

delegate void NumberChanger(int n); …​ NumberChanger nc = delegate(int x) { Console.WriteLine("Anonymous Method: {0}", x); };

The code block Console.WriteLine("Anonymous Method: {0}", x); is the body of the anonymous method.

The delegate could be called both with anonymous methods as well as named methods in the same way, i.e., by passing the method parameters to the delegate object.

For example,

[source]

nc(10);

=== Example

The following example demonstrates the concept −

[source]

using System;

delegate void NumberChanger(int n); namespace DelegateAppl {

class TestDelegate {
   static int num = 10;
public static void AddNum(int p) {
   num += p;
   Console.WriteLine("Named Method: {0}", num);
}
public static void MultNum(int q) {
   num *= q;
   Console.WriteLine("Named Method: {0}", num);
}
public static int getNum() {
   return num;
}
static void Main(string[] args) {
   //create delegate instances using anonymous method
   NumberChanger nc = delegate(int x) {
      Console.WriteLine("Anonymous Method: {0}", x);
   };
//calling the delegate using the anonymous method
nc(10);
//instantiating the delegate using the named methods
nc =  new NumberChanger(AddNum);
//calling the delegate using the named methods
nc(5);
//instantiating the delegate using another named methods
nc =  new NumberChanger(MultNum);
         //calling the delegate using the named methods
         nc(2);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Anonymous Method: 10 Named Method: 15 Named Method: 30

== C

C# allows using pointer variables in a function of code block when it is marked by the *unsafe* modifier. The *unsafe code* or the unmanaged code is a code block that uses a *pointer* variable.


=== Pointers

A *pointer* is a variable whose value is the address of another variable i.e., the direct address of the memory location. similar to any variable or constant, you must declare a pointer before you can use it to store any variable address.

The general form of a pointer declaration is −

[source]

type *var-name;

Following are valid pointer declarations −

[source]

int ip; / pointer to an integer / double *dp; / pointer to a double / float *fp; / pointer to a float / char *ch / pointer to a character */

The following example illustrates use of pointers in C#, using the unsafe modifier −

[source]

using System;

namespace UnsafeCodeApplication {

class Program {
      static unsafe void Main(string[] args) {
         int var = 20;
         int* p = &var;
         Console.WriteLine("Data is: {0} ",  var);
         Console.WriteLine("Address is: {0}",  (int)p);
         Console.ReadKey();
      }
   }
}
When the above code wass compiled and executed, it produces the following result −

[source]

Data is: 20 Address is: 99215364

Instead of declaring an entire method as unsafe, you can also declare a part of the code as unsafe. The example in the following section shows this.


=== Retrieving the Data Value Using a Pointer

You can retrieve the data stored at the located referenced by the pointer variable, using the *ToString()* method. The following example demonstrates this −

[source]

using System;

namespace UnsafeCodeApplication {

class Program {
public static void Main() {
unsafe {
   int var = 20;
   int* p = &var;
   Console.WriteLine("Data is: {0} " , var);
   Console.WriteLine("Data is: {0} " , p->ToString());
   Console.WriteLine("Address is: {0} " , (int)p);
}
         Console.ReadKey();
      }
   }
}
When the above code was compiled and executed, it produces the following result −

[source]

Data is: 20 Data is: 20 Address is: 77128984

=== Passing Pointers as Parameters to Methods

You can pass a pointer variable to a method as parameter. The following example illustrates this −

[source]

using System;

namespace UnsafeCodeApplication {

class TestPointer {
public unsafe void swap(int* p, int *q) {
   int temp = *p;
   *p = *q;
   *q = temp;
}
public unsafe static void Main() {
   TestPointer p = new TestPointer();
   int var1 = 10;
   int var2 = 20;
   int* x = &var1;
   int* y = &var2;
Console.WriteLine("Before Swap: var1:{0}, var2: {1}", var1, var2);
p.swap(x, y);
         Console.WriteLine("After Swap: var1:{0}, var2: {1}", var1, var2);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

Before Swap: var1: 10, var2: 20 After Swap: var1: 20, var2: 10

=== Accessing Array Elements Using a Pointer

In C#, an array name and a pointer to a data type same as the array data, are not the same variable type. For example, int *p and int[] p, are not same type. You can increment the pointer variable p because it is not fixed in memory but an array address is fixed in memory, and you can't increment that.

Therefore, if you need to access an array data using a pointer variable, as we traditionally do in C, or C++ ( please check: link:../cprogramming/c_pointers.html[C Pointers]), you need to fix the pointer using the *fixed* keyword.

The following example demonstrates this −

[source]

using System;

namespace UnsafeCodeApplication {

class TestPointer {
public unsafe static void Main() {
   int[]  list = {10, 100, 200};
   fixed(int *ptr = list)
/* let us have array address in pointer */
for ( int i = 0; i < 3; i++) {
   Console.WriteLine("Address of list[{0}]={1}",i,(int)(ptr + i));
   Console.WriteLine("Value of list[{0}]={1}", i, *(ptr + i));
}
         Console.ReadKey();
      }
   }
}
When the above code was compiled and executed, it produces the following result −

[source]

Address of list[0] = 31627168 Value of list[0] = 10 Address of list[1] = 31627172 Value of list[1] = 100 Address of list[2] = 31627176 Value of list[2] = 200

=== Compiling Unsafe Code

For compiling unsafe code, you have to specify the */unsafe* command-line switch with command-line compiler.

For example, to compile a program named prog1.cs containing unsafe code, from command line, give the command −

[source]

csc /unsafe prog1.cs

If you are using Visual Studio IDE then you need to enable use of unsafe code in the project properties.

To do this −

. Open *project properties* by double clicking the properties node in the Solution Explorer.


. Click on the *Build* tab.


. Select the option "*Allow unsafe code*".



== C

A *thread* is defined as the execution path of a program. Each thread defines a unique flow of control. If your application involves complicated and time consuming operations, then it is often helpful to set different execution paths or threads, with each thread performing a particular job.

Threads are *lightweight processes*. One common example of use of thread is implementation of concurrent programming by modern operating systems. Use of threads saves wastage of CPU cycle and increase efficiency of an application.

So far we wrote the programs where a single thread runs as a single process which is the running instance of the application. However, this way the application can perform one job at a time. To make it execute more than one task at a time, it could be divided into smaller threads.


=== Thread Life Cycle

The life cycle of a thread starts when an object of the System.Threading.Thread class is created and ends when the thread is terminated or completes execution.

Following are the various states in the life cycle of a thread −

. *The Unstarted State* − It is the situation when the instance of the thread is created but the Start method is not called.


. *The Ready State* − It is the situation when the thread is ready to run and waiting CPU cycle.


. *The Not Runnable State* − A thread is not executable, when
Sleep method has been calledWait method has been calledBlocked by I/O operations

. *The Dead State* − It is the situation when the thread completes execution or is aborted.



=== The Main Thread

In C#, the *System.Threading.Thread* class is used for working with threads. It allows creating and accessing individual threads in a multithreaded application. The first thread to be executed in a process is called the *main* thread.

When a C# program starts execution, the main thread is automatically created. The threads created using the *Thread* class are called the child threads of the main thread. You can access a thread using the *CurrentThread* property of the Thread class.

The following program demonstrates main thread execution −

[source]

using System; using System.Threading;

namespace MultithreadingApplication {

class MainThreadProgram {
      static void Main(string[] args) {
         Thread th = Thread.CurrentThread;
         th.Name = "MainThread";
         Console.WriteLine("This is {0}", th.Name);
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

This is MainThread

=== Properties and Methods of the Thread Class

The following table shows some most commonly used *properties* of the *Thread* class −
[%autowidth]
|===

|Sr.No.|Property & Description
|1|*CurrentContext*
Gets the current context in which the thread is executing.

|2|*CurrentCulture*
Gets or sets the culture for the current thread.

|3|*CurrentPrinciple*
Gets or sets the thread's current principal (for role-based security).

|4|*CurrentThread*
Gets the currently running thread.

|5|*CurrentUICulture*
Gets or sets the current culture used by the Resource Manager to look up culture-specific resources at run-time.

|6|*ExecutionContext*
Gets an ExecutionContext object that contains information about the various contexts of the current thread.

|7|*IsAlive*
Gets a value indicating the execution status of the current thread.

|8|*IsBackground*
Gets or sets a value indicating whether or not a thread is a background thread.

|9|*IsThreadPoolThread*
Gets a value indicating whether or not a thread belongs to the managed thread pool.

|10|*ManagedThreadId*
Gets a unique identifier for the current managed thread.

|11|*Name*
Gets or sets the name of the thread.

|12|*Priority*
Gets or sets a value indicating the scheduling priority of a thread.

|13|*ThreadState*
Gets a value containing the states of the current thread.

|===

The following table shows some of the most commonly used *methods* of the *Thread* class −
[%autowidth]
|===

|Sr.No.|Method & Description
|1|*public void Abort()*
Raises a ThreadAbortException in the thread on which it is invoked, to begin the process of terminating the thread. Calling this method usually terminates the thread.

|2|*public static LocalDataStoreSlot AllocateDataSlot()*
Allocates an unnamed data slot on all the threads. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.

|3|*public static LocalDataStoreSlot AllocateNamedDataSlot(string name)*
Allocates a named data slot on all threads. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.

|4|*public static void BeginCriticalRegion()*
Notifies a host that execution is about to enter a region of code in which the effects of a thread abort or unhandled exception might jeopardize other tasks in the application domain.

|5|*public static void BeginThreadAffinity()*
Notifies a host that managed code is about to execute instructions that depend on the identity of the current physical operating system thread.

|6|*public static void EndCriticalRegion()*
Notifies a host that execution is about to enter a region of code in which the effects of a thread abort or unhandled exception are limited to the current task.

|7|*public static void EndThreadAffinity()*
Notifies a host that managed code has finished executing instructions that depend on the identity of the current physical operating system thread.

|8|*public static void FreeNamedDataSlot(string name)*
Eliminates the association between a name and a slot, for all threads in the process. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.

|9|*public static Object GetData(LocalDataStoreSlot slot)*
Retrieves the value from the specified slot on the current thread, within the current thread's current domain. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.

|10|*public static AppDomain GetDomain()*
Returns the current domain in which the current thread is running.

|11|*public static AppDomain GetDomainID()*
Returns a unique application domain identifier

|12|*public static LocalDataStoreSlot GetNamedDataSlot(string name)*
Looks up a named data slot. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.

|13|*public void Interrupt()*
Interrupts a thread that is in the WaitSleepJoin thread state.

|14|*public void Join()*
Blocks the calling thread until a thread terminates, while continuing to perform standard COM and SendMessage pumping. This method has different overloaded forms.

|15|*public static void MemoryBarrier()*
Synchronizes memory access as follows: The processor executing the current thread cannot reorder instructions in such a way that memory accesses prior to the call to MemoryBarrier execute after memory accesses that follow the call to MemoryBarrier.

|16|*public static void ResetAbort()*
Cancels an Abort requested for the current thread.

|17|*public static void SetData(LocalDataStoreSlot slot, Object data)*
Sets the data in the specified slot on the currently running thread, for that thread's current domain. For better performance, use fields marked with the ThreadStaticAttribute attribute instead.

|18|*public void Start()*
Starts a thread.

|19|*public static void Sleep(int millisecondsTimeout)*
Makes the thread pause for a period of time.

|20|*public static void SpinWait(int iterations)*
Causes a thread to wait the number of times defined by the iterations parameter

|21|*public static byte VolatileRead(ref byte address)*
*public static double VolatileRead(ref double address)*
*public static int VolatileRead(ref int address)*
*public static Object VolatileRead(ref Object address)*
Reads the value of a field. The value is the latest written by any processor in a computer, regardless of the number of processors or the state of processor cache. This method has different overloaded forms. Only some are given above.

|22|*public static void VolatileWrite(ref byte address,byte value)*
*public static void VolatileWrite(ref double address, double value)*
*public static void VolatileWrite(ref int address, int value)*
*public static void VolatileWrite(ref Object address, Object value)*
Writes a value to a field immediately, so that the value is visible to all processors in the computer. This method has different overloaded forms. Only some are given above.

|23|*public static bool Yield()*
Causes the calling thread to yield execution to another thread that is ready to run on the current processor. The operating system selects the thread to yield to.

|===


=== Creating Threads

Threads are created by extending the Thread class. The extended Thread class then calls the *Start()* method to begin the child thread execution.

The following program demonstrates the concept −

[source]

using System; using System.Threading;

namespace MultithreadingApplication {

class ThreadCreationProgram {
public static void CallToChildThread() {
   Console.WriteLine("Child thread starts");
}
      static void Main(string[] args) {
         ThreadStart childref = new ThreadStart(CallToChildThread);
         Console.WriteLine("In Main: Creating the Child thread");
         Thread childThread = new Thread(childref);
         childThread.Start();
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

In Main: Creating the Child thread Child thread starts

=== Managing Threads

The Thread class provides various methods for managing threads.

The following example demonstrates the use of the *sleep()* method for making a thread pause for a specific period of time.

[source]

using System; using System.Threading;

namespace MultithreadingApplication {

class ThreadCreationProgram {
public static void CallToChildThread() {
   Console.WriteLine("Child thread starts");
// the thread is paused for 5000 milliseconds
int sleepfor = 5000;
   Console.WriteLine("Child Thread Paused for {0} seconds", sleepfor / 1000);
   Thread.Sleep(sleepfor);
   Console.WriteLine("Child thread resumes");
}
      static void Main(string[] args) {
         ThreadStart childref = new ThreadStart(CallToChildThread);
         Console.WriteLine("In Main: Creating the Child thread");
         Thread childThread = new Thread(childref);
         childThread.Start();
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

In Main: Creating the Child thread Child thread starts Child Thread Paused for 5 seconds Child thread resumes

=== Destroying Threads

The *Abort()* method is used for destroying threads.

The runtime aborts the thread by throwing a *ThreadAbortException*. This exception cannot be caught, the control is sent to the finally block, if any.

The following program illustrates this −

[source]

using System; using System.Threading;

namespace MultithreadingApplication {

class ThreadCreationProgram {
public static void CallToChildThread() {
try {
   Console.WriteLine("Child thread starts");
// do some work, like counting to 10
for (int counter = 0; counter <= 10; counter++) {
   Thread.Sleep(500);
   Console.WriteLine(counter);
}
      Console.WriteLine("Child Thread Completed");
   } catch (ThreadAbortException e) {
      Console.WriteLine("Thread Abort Exception");
   } finally {
      Console.WriteLine("Couldn't catch the Thread Exception");
   }
}
static void Main(string[] args) {
   ThreadStart childref = new ThreadStart(CallToChildThread);
   Console.WriteLine("In Main: Creating the Child thread");
   Thread childThread = new Thread(childref);
   childThread.Start();
//stop the main thread for some time
Thread.Sleep(2000);
//now abort the child
Console.WriteLine("In Main: Aborting the Child thread");
         childThread.Abort();
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −

[source]

In Main: Creating the Child thread Child thread starts 0 1 2 In Main: Aborting the Child thread Thread Abort Exception Couldn’t catch the Thread Exception