C# 0 The Complete Reference

a

 is not a 

B

, the cast of 

a

 to 

B

 is invalid and is prevented by the 

if

statement. However, this approach requires two steps. First, the validity of the cast must be 

confirmed. Second, the cast must be made. These two steps can be combined into one through 

the use of 

as

, as the following program shows:

// Demonstrate as.

using System;

class A {}

class B : A {}

class CheckCast {

  static void Main() {

    A a = new A();

    B b = new B();

    b = a as B; // cast, if possible

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    if(b==null)

      Console.WriteLine("The cast in b = (B) a is NOT allowed.");

    else

      Console.WriteLine("The cast in b = (B) a is allowed");

  }

}

Here is the output, which is the same as before:

The cast in b = (B) a is NOT allowed.

In this version, the 

as

 statement checks the validity of the cast and then, if valid, performs 

the cast, all in one statement.

Using typeof

Although useful in their own ways, the