C# 0 The Complete Reference

OnLButtonClick( )

 can be called, and all registered handlers will be notified.

Although developing a Windows program that demonstrates this approach is beyond 

the scope of this chapter, it is possible to give an idea of how such an approach would work. 

The following program creates an event handler that processes keystrokes. The event is 

called

KeyPress

, and each time a key is pressed, the event is fired by calling 

OnKeyPress( )

.

Notice that .NET-compatible events are created and that lambda expressions provide the 

event handlers.

// A keypress event example.

using System;

// Derive a custom EventArgs class that holds the key.

class KeyEventArgs : EventArgs {

  public char ch;

}

// Declare a delegate type for an event.

delegate void KeyHandler(object source, KeyEventArgs arg);

// Declare a keypress event class.

class KeyEvent {

  public event KeyHandler KeyPress;

  // This is called when a key is pressed.

  public void OnKeyPress(char key) {

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    KeyEventArgs k = new KeyEventArgs();

    if(KeyPress != null) {

      k.ch = key;

      KeyPress(this, k);

    }

  }

}

// Demonstrate KeyEvent.

class KeyEventDemo {

  static void Main() {

    KeyEvent kevt = new KeyEvent();

    ConsoleKeyInfo key;

    int count = 0;

    // Use a lambda expression to display the keypress.

    kevt.KeyPress += (source, arg) =>

      Console.WriteLine(" Received keystroke: " + arg.ch);

    // Use a lambda expression to count keypresses.

    kevt.KeyPress += (source, arg) =>

      count++; // count is an outer variable

    Console.WriteLine("Enter some characters. " +

                      "Enter a period to stop.");

    do {

      key = Console.ReadKey();

      kevt.OnKeyPress(key.KeyChar);

    } while(key.KeyChar != '.');

    Console.WriteLine(count + " keys pressed.");

  }

}

Here is a sample run:

Enter some characters. Enter a period to stop.

t Received keystroke: t

e Received keystroke: e

s Received keystroke: s

t Received keystroke: t

. Received keystroke: .

5 keys pressed.

The program begins by deriving a class from