Study Notes Delegate in C#

Explanation:

Delegate is like hook in C++.

First define a delegate.

Then attach some methods to the delegate.

And then activate delegate when some events occur; delegate will run the methods attached to it.

Example:

class TestDelegate

{

         //define methods which will be attached to delegate

         public static void ShowMsgStatic()

         {

                   System.Console.WriteLine("TestDelegate.ShowMsgStatic()");

         }

         public void ShowMsgNonStatic()

         {

                   System.Console.WriteLine("TestDelegate.ShowMsgNonStatic()");

         }

 

}

class ClassDelegate

{

         //define a delegate

         public delegate void Action();

         private static Action  theAction;

         public static Action Tester

         {

                   get { return theAction; }

                   set { theAction = value; }

         }

 

         public static void run()

         {

                   if (theAction != null)

                            theAction();

         }

}

 

class ClassCaller

{

    public void Execute()

    {

        //attach some methods to delegate

        ClassDelegate.Tester += new ClassDelegate.Action(TestDelegate.ShowMsgStatic);

        ClassDelegate.Tester += new ClassDelegate.Action(new TestDelegate().ShowMsgNonStatic);

 

        //Call delegate: system will run TestDelegate.ShowMsgStatic and TestDelegate.ShowMsgNonStatic in sequence

        ClassDelegate.run();

    }

}

Notes:

1.         Static or non static methods can be attached to delegate

2.         Usually, delegate is attached and called in different place. (This example just want to demonstrate simply, so includes attach and call in one class)

3.         The methods which will be called by delegate can have parameters and return value, the only constraint is methods should have the same declaration, like:

public int ShowMsg (string str) {return str.Length;}    //define method

public delegate int ActionWithParam(string str);        //define delegate

int i = theActionWithParam("123");                      //call delegate

 

4.         If multiple methods with return value are attached to one delegate, when call the delegate, system just return the value returned by method last called.

5.         Be able to unattach methods from delegate:

ClassDelegate.Action act  = new ClassDelegate.Action(TestDelegate.ShowMsgStatic);
ClassDelegate.Tester += act;
ClassDelegate.run();
ClassDelegate.Tester -= act;           //unattach the method

 

6.         The sequence of calling methods is the same with the sequence of attaching methods to delegate, if the methods are attached in one class. But developers can’t decide the sequence if there are multiple classed in which methods are attached to delegate.

7.         Delegate is used like command pattern, it can store some handle logic at first, and activate them when some events occur.