C# 0 The Complete Reference

The using Statement

In addition to the 

using

directive

 discussed earlier, 

using

 has a second form that is called the 

using

statement.

 It has these general forms:

using (

obj

) {

   // use 

obj

}

using (

type obj

 = 

initializer

) {

  // use 

obj

}

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Here, 

obj

 is an expression that must evaluate to an object that implements the 

System.IDisposable

 interface. It specifies a variable that will be used inside the 

using

block. In the first form, the object is declared outside the 

using

 statement. In the second 

form, the object is declared within the 

using

 statement. When the block concludes, the 

Dispose( )

 method (defined by the 

System

.

IDisposable

 interface) will be called on 

obj

.

Dispose( )

 is called even if the 

using

 block ends because of an exception. Thus, a 

using

statement provides a means by which objects are automatically disposed when they are no 

longer needed. Remember, the 

using

 statement applies only to objects that implement the 

System.IDisposable

interface.

Here is an example of each form of the 

using

 statement:

// Demonstrate using statement.

using System;

using System.IO;

class UsingDemo {

  static void Main() {

    try {

      StreamReader sr = new StreamReader("test.txt");

      // Use object inside using statement.

      using(sr) {

        // ...

      }

    } catch(IOException exc) {

       // ...

    }

    try {

      // Create a StreamReader inside the using statement.

      using(StreamReader sr2 = new StreamReader("test.txt")) {

        // ...

      }

    } catch(IOException exc) {

      // ...

    }

  }

}

The class 

StreamReader

 implements the 

IDisposable

 interface (through its base class 

TextReader

). Thus, it can be used in a 

using

 statement. When the 

using

 statement ends, 

Dispose( )

 is automatically called on the stream variable, thus closing the stream.

As the preceding example illustrates, 

using

 is particularly useful when working with 

files because the file is automatically closed at the end of the 

using

 block, even if the block 

ends because of an exception. As a result, closing a file via 

using

 often simplifies file-

handling code.

extern

The

extern

 keyword has two uses. Each is examined here.

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