Csharp 简明教程

using System;

namespace HelloWorldApplication {

   class HelloWorld {

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

Hello World

using System;

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

}//end class Rectangle

using System;

namespace DataTypeApplication {

   class Program {

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

Size of int: 4

type* identifier;

char* cptr;
int* iptr;

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();

      }
   }
}

75
53.005
2345.7652
True

<data_type> <variable_list>;

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

int i = 100;

variable_name = value;

<data_type> <variable_name> = value;

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'. */

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();
      }
   }
}

a = 10, b = 20, c = 30

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

int g = 20;

10 = 20;

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

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

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

using System;

namespace EscapeChar {

   class Program {

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

Hello   World

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

const <data_type> <constant_name> = value;

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();
        }
    }
}

Enter Radius:
3
Radius: 3, Area: 28.27431

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]

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]

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]

When the above code is compiled and executed, it produces the following result −

[source]

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]

When the above code is compiled and executed, it produces the following result −

[source]

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]

When the above code is compiled and executed, it produces the following result −

[source]

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]

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]

=== Calling Methods in C

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

[source]

When the above code is compiled and executed, it produces the following result −

[source]

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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

The following example demonstrates use of nullable data types −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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]

=== 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]

=== Assigning Values to an Array

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

[source]

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

[source]

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

[source]

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

[source]

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]

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]

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

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

==== String Contains String

[source]

When the above code is compiled and executed, it produces the following result −

[source]

==== Getting a Substring

[source]

When the above code is compiled and executed, it produces the following result −

[source]

==== Joining Strings

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

The following program shows the use of the structure −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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]

=== Example

The following example demonstrates use of enum variable −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

Consider a base class Shape and its derived class Rectangle −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

=== Example

The following example demonstrates implementation of the above interface −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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

[source]

The following program demonstrates use of namespaces −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

We could have written the fully qualified name as −

[source]

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]

When the above code is compiled and executed, it produces the following result −

[source]

=== Nested Namespaces

You can define one namespace inside another namespace as follows −

[source]

You can access members of nested namespace by using the dot (.) operator as follows −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

The following program illustrates this −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== Example 2

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

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Example 3

This example replaces extra white space −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

== 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]

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

[source]

[%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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

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]

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]

=== 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]

For example,

[source]

The following example demonstrates the attribute −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

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]

When you try to compile the program, the compiler gives an error message stating −

[source]

=== 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]

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]

=== Applying the Custom Attribute

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

[source]

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]

The following program demonstrates this −

[source]

When it is compiled and run, it displays the name of the custom attributes attached to the class MyClass −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

=== Example

The following example demonstrates use of properties −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Abstract Properties

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

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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]

=== 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]

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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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

[source]

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


=== Example

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== Generic Delegates

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

[source]

The following example shows use of this delegate −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

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]

=== Example

The following example demonstrates the concept −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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]

Following are valid pointer declarations −

[source]

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

[source]

When the above code wass compiled and executed, it produces the following result −

[source]

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]

When the above code was compiled and executed, it produces the following result −

[source]

=== Passing Pointers as Parameters to Methods

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

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code was compiled and executed, it produces the following result −

[source]

=== 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]

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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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]

When the above code is compiled and executed, it produces the following result −

[source]