C# 0 The Complete Reference

PART I

C h a p t e r   1 5 :  

D e l e g a t e s ,   E v e n t s ,   a n d   L a m b d a   E x p r e s s i o n s  

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PART IPART I

  public event MyEventHandler SomeEvent;

  // This is called to fire the event.

  public void OnSomeEvent() {

    if(SomeEvent != null)

      SomeEvent();

  }

}

class X {

  int id;

  public X(int x) { id = x; }

  // This is an instance method that will be used as an event handler.

  public void Xhandler() {

    Console.WriteLine("Event received by object " + id);

  }

}

class EventDemo3 {

  static void Main() {

    MyEvent evt = new MyEvent();

    X o1 = new X(1);

    X o2 = new X(2);

    X o3 = new X(3);

    evt.SomeEvent += o1.Xhandler;

    evt.SomeEvent += o2.Xhandler;

    evt.SomeEvent += o3.Xhandler;

    // Fire the event.

    evt.OnSomeEvent();

  }

}

The output from this program is shown here:

Event received by object 1

Event received by object 2

Event received by object 3

As the output shows, each object registers its interest in an event separately, and each 

receives a separate notification.

Alternatively, when a 

static

 method is used as an event handler, events are handled 

independently of any object, as the following program shows:

/* A class receives the notification when

   a static method is used as an event handler. */

using System;

// Declare a delegate type for an event.

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424

 

P a r t   I :  

T h e   C #   L a n g u a g e

delegate void MyEventHandler();

// Declare a class that contains an event.

class MyEvent {

  public event MyEventHandler SomeEvent;

  // This is called to fire the event.

  public void OnSomeEvent() {

    if(SomeEvent != null)

      SomeEvent();

  }

}

class X {

  /* This is a static method that will be used as

     an event handler. */

  public static void Xhandler() {

    Console.WriteLine("Event received by class.");

  }

}

class EventDemo4 {

  static void Main() {

    MyEvent evt = new MyEvent();

    evt.SomeEvent += X.Xhandler;

    // Fire the event.

    evt.OnSomeEvent();

  }

}

The output from this program is shown here:

Event received by class.

In the program, notice that no object of type