Event Handling in .NET Using C#

Event Handlers in C#

An event handler in C# is a delegate with a special signature, given below.

public delegate void MyEventHandler(object sender, MyEventArgs e);

The first parameter (sender) in the above declaration specifies the object that fired the event. The second parameter (e) of the above declaration holds data that can be used in the event handler. The class MyEventArgs is derived from the class EventArgs. EventArgs is the base class of more specialized classes, like MouseEventArgs, ListChangedEventArgs, etc. For GUI event, you can use objects of these specialized EventArgs classes without creating your own specialized EventArgs classes. However, for non GUI event, you need to create your own specialized EventArgs class to hold your data that you want to pass to the delegate object. You create your specialized EventArgs class by deriving from EventArgs class.

public class MyEventArgs EventArgs{
    public string m_myEventArgumentdata;
}

In case of event handler, the delegate object is referenced using the key word event as follows

public event MyEventHandler MyEvent;

Now, we will set up two classes to see how this event handling mechanism works in .Net framework. The step 2 in the discussion of delegates requires that we define methods with the exact same signature as that of the delegate declaration. In our example, class A will provide event handlers (methods with the same signature as that of the delegate declaration). It will create the delegate objects (step 3 in the discussion of delegates) and hook up the event handler. Class A will then pass the delegate objects to class B. When an event occurs in Class B, it will execute the event handler method in Class A.

using System;
//Step 1 Create delegate object
public delegate void MyHandler1(object sender,MyEventArgs e);
public delegate void MyHandler2(object sender,MyEventArgs e);
//Step 2 Create event handler methods
class A {
   public const string m_id="Class A";
   public void OnHandler1(object sender,MyEventArgs e){
       Console.WriteLine("I am in OnHandler1 and MyEventArgs is {0}", e.m_id);
   }
   public void OnHandler2(object sender,MyEventArgs e){
       Console.WriteLine("I am in OnHandler2 and MyEventArgs is {0}", e.m_id);
   }
   //Step 3 create delegates, plug in the handler and register with the object that will fire the events
   public A(B b) {
       MyHandler1 d1=new MyHandler1(OnHandler1);
       MyHandler2 d2=new MyHandler2(OnHandler2);
       b.Event1 +=d1;
       b.Event2 +=d2;
   }
}
//Step 4 Calls the encapsulated methods through the delegates (fires events)
class B {
   public event MyHandler1 Event1;
   public event MyHandler2 Event2;
   public void FireEvent1(MyEventArgs e) {
       if(Event1 != null){
           Event1(this,e);
       }
   }
   public void FireEvent2(MyEventArgs e){
       if(Event2 != null){
           Event2(this,e);
       }
   }
}
public class MyEventArgs : EventArgs{
   public string m_id;
}
public class Driver {
   public static void Main(){
       B b= new B();
       A a= new A(b);
       MyEventArgs e1=new MyEventArgs();
       MyEventArgs e2=new MyEventArgs();
       e1.m_id ="Event args for event 1";
       e2.m_id ="Event args for event 2";
       b.FireEvent1(e1);
       b.FireEvent2(e2);    
   }
}

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