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Asynchronous Calls in .NET : Page 2

Invoking methods asynchronously involves a substantial amount of infrastructure: keeping track of methods in progress, multithreading, and error handling. Fortunately, .NET provides a standard method for asynchronous invocation that's consistent and available to every component and client.




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What Delegates Are, Actually
To the programmer, a delegate is nothing more than a type-safe method reference. The delegate (as the name implies) is used to delegate the act of calling a method on an object (or a static method on a class) from the client to the delegate class. For example, consider a Calculator class:

   public class Calculator
      public int Add(int num1,int num2)
         return num1+num2;
      public int Subtract(int num1,int num2)
         return num1-num2;
      //Other methods
Instead of calling the Add() method directly, you can define a delegate called BinaryOperation:

   public delegate int BinaryOperation( 
      int num1,int num2);
You can use BinaryOperation to invoke the method:

   Calculator calc = new Calculator();
   BinaryOperation oppDel;
   oppDel = new BinaryOperation(calc.Add); 
   int result = 0;
   result = oppDel(2,3);
   Debug.Assert(result == 5);
By default, when you use a delegate to invoke methods, the delegate blocks the caller until all target methods return. In the example just shown, the caller is blocked until Add() returns. However, the delegate can also be used to invoke its target method asynchronously. The truth is that there isn't really anything special about delegates because delegates are actually compiled to classes. When you define a delegate type, the compiler converts the delegate declaration to a sophisticated signature-specific class definition, and inserts that class instead of the delegate definition. For example, instead of this delegate definition:

   public delegate int BinaryOperation(int num1,int num2);
The compiler generates this class definition:

   public class BinaryOperation: System.MulticastDelegate
      public BinaryOperation(Object target,
         int methodPtr)
      public virtual int Invoke(int num1,int num2)
      public virtual IAsyncResult BeginInvoke(
         int num1,int num2, AsyncCallback callback,
         object asyncState)
      public virtual int EndInvoke(
         IAsyncResult result)
You can use the delegate simply to invoke a method, such as in this code.

   Calculator calc = new Calculator();
   BinaryOperation oppDel;
   oppDel = new BinaryOperation(calc.Add);
When you do that, the compiler converts the call to oppDel(2,3) to a call to the Invoke() method. The Invoke() method blocks the caller, executes the method on the caller's thread, and returns control to the caller.

The compiler-generated BinaryOperation class derives from a class called MulticastDelegate defined in the System namespace. The compiler also declares two methods used to manage asynchronous method invocation. These methods are BeginInvoke() and EndInvoke(), and the proper use of them is the subject of this article.

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