Csharp 简明教程

C

异常是在执行程序期间出现的问题。C# 异常是对程序运行期间出现的特殊情况（如尝试除以零）的响应。

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.

异常提供了一种将程序某一部分的控制权转移到程序另一部分的方法。C# 异常处理建立在四个关键字的基础上： try 、 catch 、 finally 和 throw 。

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.

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

  2. 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.

  3. 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.

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

Syntax

假设一个块引发了异常，那么该方法使用 try 和 catch 关键字的组合捕获了一个异常。try/catch 块周围放有可能会生成异常的代码。try/catch 块中的代码称为受保护代码，而使用 try/catch 的语法如下所示 −

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 −

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
}

你可以列出多个 catch 语句，以便在你 try 块在不同情况下引发多个异常时捕获不同类型的异常。

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# 异常由类表示。C# 中的异常类主要是直接或间接地派生自 System.Exception 类。派生自 System.Exception 类的某些异常类是 System.ApplicationException 和 * System.SystemException * 类。

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.

System.ApplicationException 类支持应用程序生成的异常。因此，程序员定义的异常应派生自此类。

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

System.SystemException 类是所有预定义的系统异常的基类。

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

下表提供了从 Sytem.SystemException 类派生的某些预定义异常类 −

The following table provides some of the predefined exception classes derived from the Sytem.SystemException class −

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# 以尝试和捕获块的形式提供了一种结构化的异常处理解决方案。使用这些块可以将程序核心语句与错误处理语句分开。

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.

这些错误处理块使用 try 、 catch 和 finally 关键字实现。以下是除以零条件发生时引发异常的一个示例 −

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 −

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();
      }
   }
}

编译并执行上述代码后，将产生以下结果 −

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

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

Creating User-Defined Exceptions

您还可以定义自己的异常。用户定义的异常类派生自 Exception 类。以下示例对此进行了演示 −

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

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);
      }
   }
}

编译并执行上述代码后，将产生以下结果 −

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

TempIsZeroException: Zero Temperature found

Throwing Objects

如果某个对象是直接或间接派生自 System.Exception 类的，则可以引发该对象。您可以在 catch 块中使用 throw 语句将当前对象作为 −

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 −

Catch(Exception e) {
   ...
   Throw e
}