Chapter 8: Delegates, Events, and Namespaces

Declaring a Delegate

A delegate declaration consists of four parts:

• The keyword delegate

• Followed by the return type of the function to be called

• Followed by a name that will be generic to the function to be called

• Followed by a parameter list

In the following example, the delegate called MyDelegate can call any function that returns an int and is passed a single string:

delegate int MyDelegate(string s);

You can use delegates to fire a single method, or you may multicast methods. You may initialize a new delegate using static or instance methods, using the following syntax:

MyDelegate Del1 = new MyDelegate(DoSomething);

You can also initialize a delegate using this syntax for multicasting:

MyDelegate Multicast = null;
m += new MyDelegate(DoSomething);
m += new MyDelegate(DoSomething2);

After you have initialized your delegate with values, you use it by calling the delegate, as follows:

del1(MyString);

In the multicast case, you call the delegate as follows:

m(MyString);

This will result in the DoSomething method being called, followed by the DoSomething2 method.

To remove a method from the delegate list, use this syntax:

m -= new MyDelegate(DoSomething2);

This will remove DoSomething2 from the delegate known as m.

You can also iterate through a list of all the delegates via the following code:

foreach(Delegate D in MyInstanceDelegate(GetInvocationList())
{
      (cast)D(params);
}

It is also important to realize that every time you declare a delegate, a new class is generated. This class does the work for you under the hood.

When to Use a Delegate

You should consider using a delegate in the following situations:

• You want to use a C-style function pointer.

• You desire a single callback invocation.

• You want the callback function registered at construction time rather than via a method call.

You should use an interface if the callback function involves complex behavior or when using Remoting.

The following example demonstrates using delegates.

Code Example: Delegates

using System;

namespace Client.Chapter_8___Delegates_Events_and_Namespaces
{
      class Delegates
      {
           //Creates a method pointer
            delegate int MyDelegate(string s);
            static void Main(string[] args)
            {
                  MyDelegate Del1 = new MyDelegate(DoSomething);
                  MyDelegate Del2 = new MyDelegate(DoSomething2);
                  string MyString = "Hello World";

                  Del1(MyString);
                  Del2(MyString);

                 //Or you can multicast delegates by doing this
                  MyDelegate Multicast = null;

                  Multicast += new MyDelegate(DoSomething);
                  Multicast += new MyDelegate(DoSomething2);

                 //Both DoSomething & DoSomething2 will be fired
                 //in the order they are added to the delegate
                  Multicast(MyString);
                  Multicast -= new MyDelegate(DoSomething2);
            }
            static int DoSomething(string s)
            {
                  return 0;
            }
            static int DoSomething2(string s)
            {
                  return 0;
            }
      }
}