Delegates Inside

CodeProject


Delegates are one of most complex and simplest concept in .Net ;) .

Delegates in c# are defained as Function Pointers. Those functions can be an instance function or a static function. In case of instance function delegeate store the reference of that object in its property called "Target".

Once an instance function is added to a deletegate then that function will always be invoked by delegate event if you set instance to null................

Lets take an example :

---

using System;

static class Program
{
[STAThread]
static void Main()
{

EventHandelerClass handeler = new EventHandelerClass("My Text");
delShow showMe = new delShow(handeler.OnShow);
handeler.Text = "Changed ";
handeler = null;  // set to null 
GC.Collect(); // Make sure it is released 
Console.WriteLine(showMe(" From Delegate"));
Console.ReadLine();

}
}


public delegate String delShow(String msg);

public class EventHandelerClass
{
public String Text
{
get;
set;
}
public EventHandelerClass(string text)
{
Text = text;
}

public String OnShow(String msg)
{
return this.Text + msg;
}


}

---

Above program will display message "My Text From Delegate" even if handeler is set to null.

This is because when you add a instance method to a delegate it store reference of that instance in its "Target" property. When we set handeler = null actually we are saying now "handeler" will no longer point to the address but delegate "showMe" is still pointing to same address ( ref How reference types are stored ? ).

Whole idea is once we have assign a function pointer to a delegate or event it will always call it ; simply setting instance to null will not remove the handler. To remove handler we need to either use "-=" or "MulticastDelegate.Remove" method.

I faced this problem while implementing MVP in one of my project where I have disposed a consumer form but the event handler was still called. in my next article I will try to explain MVP implementation in detail.