C# 0 The Complete Reference

MyEventArgs

 is derived from 

EventArgs

.

MyEventArgs

 adds just one 

field of its own: 

EventNum

. The event handler delegate 

MyEventHandler

 now takes the 

two parameters required by the .NET Framework. As explained, the first is an object 

reference to the generator of the event. The second is a reference to 

EventArgs

 or a class 

derived from 

EventArgs

. The event handlers in the 

X

 and 

Y

 classes, 

Handler( )

, also have 

the same types of parameters.

Inside

MyEvent

, a 

MyEventHandler

 called 

SomeEvent

 is declared. In the 

OnSomeEvent( ) 

method,

SomeEvent

 is called with the first argument being 

this

, and the second argument 

being a 

MyEventArgs

 instance. Thus, the proper arguments are passed to 

MyEventHandler

to fulfill the requirements for .NET compatibility.

Use EventHandler

For many events, the 

EventArgs

 parameter is unused. To help facilitate the creation of code in 

these situations, the .NET Framework includes a built-in delegate type called 

EventHandler

,

which can be used to declare event handlers in which no extra information is needed. Here 

is an example that uses 

EventHandler

:

// Use the built-in EventHandler delegate.

using System;

// Declare a class that contains an event.

class MyEvent {

  public event EventHandler SomeEvent; // uses EventHandler delegate

  // This is called to fire SomeEvent.

  public void OnSomeEvent() {

    if(SomeEvent != null)

      SomeEvent(this, EventArgs.Empty);

  }

}

class EventDemo7 {

  static void Handler(object source, EventArgs arg) {

    Console.WriteLine("Event occurred");

    Console.WriteLine("Source is " + source);

  }

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