Programming .NET Components, 2nd Edition

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. ...