C# 0 The Complete Reference

this

 by the calling code. The 

EventArgs

 parameter 

contains additional information and can be ignored if it is not needed.

The

EventArgs

 class itself does not contain fields that you use to pass additional data to 

a handler. Instead, 

EventArgs

 is used as a base class from which you will derive a class that 

contains the necessary fields. 

EventArgs

 does include one 

static

 field called 

Empty

, which is 

an

EventArgs

 object that contains no data.

Here is an example that creates a .NET-compatible event:

// A .NET-compatible event.

using System;

// Derive a class from EventArgs.

class MyEventArgs : EventArgs {

  public int EventNum;

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PART IPART I

}

// Declare a delegate type for an event.

delegate void MyEventHandler(object source, MyEventArgs arg);

// Declare a class that contains an event.

class MyEvent {

  static int count = 0;

  public event MyEventHandler SomeEvent;

  // This fires SomeEvent.

  public void OnSomeEvent() {

    MyEventArgs arg = new MyEventArgs();

    if(SomeEvent != null) {

      arg.EventNum = count++;

      SomeEvent(this, arg);

    }

  }

}

class X {

  public void Handler(object source, MyEventArgs arg) {

    Console.WriteLine("Event " + arg.EventNum +

                      " received by an X object.");

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

    Console.WriteLine();

  }

}

class Y {

  public void Handler(object source, MyEventArgs arg) {

    Console.WriteLine("Event " + arg.EventNum +

                      " received by a Y object.");

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

    Console.WriteLine();

  }

}

class EventDemo6 {

  static void Main() {

    X ob1 = new X();

    Y ob2 = new Y();

    MyEvent evt = new MyEvent();

    // Add Handler() to the event list.

    evt.SomeEvent += ob1.Handler;

    evt.SomeEvent += ob2.Handler;

    // Fire the event.

    evt.OnSomeEvent();

    evt.OnSomeEvent();

  }

}

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Here is the output:

Event 0 received by an X object.

Source is MyEvent

Event 0 received by a Y object.

Source is MyEvent

Event 1 received by an X object.

Source is MyEvent

Event 1 received by a Y object.

Source is MyEvent

In this example,