Section 7.2. Revisiting Delegates

7.2. Revisiting Delegates

As explained in Chapter 6, 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 argument1,int argument2)        {           return argument1 + argument2;        }        public int Subtract(int argument1,int argument2)        {           return argument1 - argument2;        }        //Other methods     }

Instead of calling the Add( ) method directly, you can define a delegate called BinaryOperation:

     public delegate int BinaryOperation(int argument1,int argument2);

and use BinaryOperation to invoke the method:

     Calculator calculator = new Calculator(  );     BinaryOperation oppDel  = calculator.Add;//Can use += as well     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, for this delegate definition:

     public delegate int BinaryOperation(int argument1,int argument2);

the compiler generates this class definition:

     public sealed class BinaryOperation : MulticastDelegate     {         public BinaryOperation(Object target,int methodPtr)         {...}         public virtual int Invoke(int argument1,int argument2)         {...}         public virtual IAsyncResult BeginInvoke(int argument1,int argument2,                                               AsyncCallback callback,object asyncState)         {...}         public virtual int EndInvoke(IAsyncResult result)         {...}     }

When you use the delegate simply to invoke a method, such as in this code:

     Calculator calculator = new Calculator(  );     BinaryOperation oppDel = calculator.Add;     oppDel(2,3);

or, in Visual Basic 2005:

     Dim calculator As New Calculator(  )     Dim oppDel As BinaryOperation     oppDel = New BinaryOperation(AddressOf calculator.Add)     oppDel(2, 3)

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, which is defined in the System namespace. MulticastDelegate provides the implementation of the internal list of delegates every delegate has, and it is in turn derived from the abstract class Delegate (described in the previous chapter).

The compiler also declares two methods that manage asynchronous method invocation. These methods are BeginInvoke( ) and EndInvoke( ), and their proper use is the subject of this chapter.