C# 2008 Programmer's Reference

Replace the original TimesUp event statement with the following statement, which uses the AlarmClockEventHandler class:

//---public event EventHandler TimesUp;---
public event AlarmClockEventHandler TimesUp;

Add a Message property to the class so that users of this class can set a message that will be returned by the event when the time is up:

public string Message { get; set; }

Modify the onTimesUp virtual method by changing its parameter type to the new AlarmClockEventArgs class:

protected virtual void onTimesUp(AlarmClockEventArgs e) {
 if (TimesUp != null) TimesUp(this, e);
}

Finally, modify the t_Elapsed event handler so that when you now call the onTimesUp() method, you pass in an instance of the AlarmClockEventArgs class containing the message you want to pass back to the event handler:

void t_Elapsed(object sender, ElapsedEventArgs e) {
 if (DateTime.Now >= this.AlarmTime) {
  onTimesUp(new AlarmClockEventArgs(this.Message));
  t.Stop();
 }
}

Here's the complete program: using System;

using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Timers;
public class AlarmClockEventArgs : EventArgs {
 public AlarmClockEventArgs(string Message) {
  this.Message = Message;
 }
 public string Message { get; set; }
}
public delegate void AlarmClockEventHandler(object sender, AlarmClockEventArgs e);
class AlarmClock {
 Timer t;
 public event AlarmClockEventHandler TimesUp;
 protected virtual void onTimesUp(AlarmClockEventArgs e) {
  if (TimesUp != null) TimesUp(this, e);
 }
 public DateTime AlarmTime { get; set; }
 public string Message { get; set; }
 public AlarmClock() {
  t = new Timer(100);
  t.Elapsed += new ElapsedEventHandler(t_Elapsed);
 }
 public void Start() {
  t.Start();
 }
 void t_Elapsed(object sender, ElapsedEventArgs e) {
  if (DateTime.Now >= this.AlarmTime) {
   onTimesUp(new AlarmClockEventArgs(this.Message));
   t.Stop();
  }
 }
}

With the modified AlarmClock class, your program will now look like this:

namespace Events {
 class Program {
  static void c_TimesUp(object sender, AlarmClockEventArgs e) {
   Console.WriteLine(DateTime.Now.ToShortTimeString() + ': ' + e.Message);
  }
  static void Main(string[] args) {
   AlarmClock c = new AlarmClock() {
    //---alarm to sound off at 16 May 08, 9.50am---
    AlarmTime = new DateTime(2008, 5, 16, 09, 50, 0, 0),
    Message = 'Meeting with customer.'
   };
   c.TimesUp += new AlarmClockEventHandler(c_TimesUp);
   c.Start();
   Console.ReadLine();
  }
 }
}

Figure 7-10 shows the output when the AlarmClock fires the TimesUp event.

Figure 7-10

Summary

This chapter discussed what delegates are and how you can use them to invoke other functions, as well as how you can use delegates to implement callbacks so that your application is more efficient and responsive. One direct application of delegates is events, which make GUI operating systems such as Windows possible. One important difference between delegates and events is that you cannot assign a delegate to an event by using the = operator.