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Using Delegates to Implement Event Handling

by Jesse Liberty, author of Programming C# 3.0, 5th Edition

06/23/2003

One of the key aspects of C# programming in particular, and .NET programming in general, is using delegates to handle events. In Programming C#, 3rd Edition, I approach teaching delegates and events somewhat differently than I had in previous editions.

This article will focus on one aspect of delegates: how they are used to implement event handling. It is important to understand that while delegates are a general-purpose mechanism for calling methods indirectly, their principal uses in .NET are for a) implementing events and b) implementing call-back methods.

To get a sense of how delegates are used to implement events, we'll look at the implementation of a custom event.

Implementing a Custom Event

In C#, any object can publish a set of events to which other classes can subscribe. When the publishing class raises an event, all the subscribed classes are notified. This design is a form of the Observer Pattern described in the seminal work Design Patterns, by Gamma, et al. (Addison Wesley, 1995). Gamma describes the intent of this pattern: "Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically."

With this mechanism, your object can say "Here are things I can notify you about," and other classes might sign up, saying "Yes, let me know when that happens." For example, a button might notify any number of interested observers when it is clicked. The button is called the "publisher," because the button publishes the Click event, and the other classes are the subscribers, because they subscribe to the Click event.

As a second example, a Clock might notify interested classes whenever the time changes by one second. The Clock class could simply print the time rather than raising an event, so why bother with the indirection of using delegates? The advantage of the publish/subscribe idiom is that any number of classes can be notified when an event is raised. The subscribing classes do not need to know how the Clock works, and the Clock does not need to know what they are going to do in response to the event.

Programming C#

Related Reading

Programming C#
By Jesse Liberty

The publisher and the subscribers are decoupled by the delegate. This is highly desirable; it makes for more flexible and robust code. The Clock can change how it detects time without breaking any of the subscribing classes. The subscribing classes can change how they respond to time changes without breaking the Clock. The two classes spin independently of one another, and that makes for code that is easier to maintain.

A method that handles an event is called an event handler. You can declare your event handlers as you would any other delegate. By convention, event handlers in the .NET Framework return void and take two parameters: The first parameter is the "source" of the event; that is, the publishing object. The second parameter is an object derived from EventArgs. It is recommended that your event handlers follow this design pattern.

EventArgs is the base class for all event data. Other than its constructor, the EventArgs class inherits all of its methods from Object, though it does add a public static field, named empty, which represents an event with no state (to allow for the efficient use of events with no state). The EventArgs-derived class contains information about the event.

Suppose you want to create a Clock class that uses delegates to notify potential subscribers whenever the local time changes value by one second. Call this delegate SecondChangeHandler.

The declaration for the SecondChangeHandler delegate is:

public delegate void SecondChangeHandler(
    object clock, 
    TimeInfoEventArgs timeInformation
    );

This delegate will encapsulate any method that returns void and that takes two parameters. The first parameter is an object that represents the clock (the object raising the event) and the second parameter is an object of type TimeInfoEventArgs that will contain useful information for anyone interested in this event.

TimeInfoEventArgs is defined as follows:

public class TimeInfoEventArgs : EventArgs
{
     public TimeInfoEventArgs(int hour, int minute, int second)
     {
         this.hour = hour;
         this.minute = minute;
         this.second = second;
     }
     public readonly int hour;
     public readonly int minute;
     public readonly int second;
}

The TimeInfoEventArgs object will have information about the current hour, minute, and second. It defines a constructor and three public, readonly integer variables.

In addition to its delegate, a Clock has three member variables, hour, minute, and second, as well as a single method, Run():

public void Run()
{
    for(;;)
    {
        // sleep 10 milliseconds
        Thread.Sleep(10);
        
        // get the current time
        System.DateTime dt = System.DateTime.Now;

        // if the second has changed
        // notify the subscribers
        if (dt.Second != second)
        {
            // create the TimeInfoEventArgs object
            // to pass to the subscriber
    TimeInfoEventArgs timeInformation = 
      new TimeInfoEventArgs(
      dt.Hour,dt.Minute,dt.Second);

            // if anyone has subscribed, notify them
            if (OnSecondChange != null)
            {
                OnSecondChange(this,timeInformation);
            }
        }
        // update the state
        this.second = dt.Second;
        this.minute = dt.Minute;
        this.hour = dt.Hour;
    }
}

Run creates an infinite for loop that periodically checks the system time. If the time has changed from the Clock object's current time, it notifies all of its subscribers and then updates its own state.

The first step is to sleep for 10 milliseconds:

Thread.Sleep(10);

After sleeping for 10 milliseconds, the method checks the current time:

System.DateTime dt = System.DateTime.Now;

About every 100 times it checks, the second will have incremented. The method notices that change and notifies its subscribers. To do so, it first creates a new TimeInfoEventArgs object:

if (dt.Second != second)
{
   TimeInfoEventArgs timeInformation = 
      new TimeInfoEventArgs(dt.Hour,dt.Minute,dt.Second);

It then notifies the subscribers by firing the OnSecondChange event (the if statement checks that the value is not null, ensuring that there are subscribers before calling OnSecondChange).

   if (OnSecondChange != null)
   {
      OnSecondChange(this,timeInformation);
   }

You will remember that OnSecondChange takes two arguments: the source of the event and the object derived from EventArgs. In the code snippet, you see that the clock's this reference is passed because the clock is the source of the event. The second parameter is the TimeInfoEventArgs object timeInformation, created on the line above.

Raising the event will invoke whatever methods have been registered with the Clock class through the delegate. We'll examine this in a moment.

Once the event is raised, you update the state of the Clock class:

this.second = dt.Second;
this.minute = dt.Minute;
this.hour = dt.Hour;

All that is left is to create classes that can subscribe to this event. You'll create two; your first will be the DisplayClock class. The job of DisplayClock is not to keep track of time, but rather to display the current time to the console.

The example simplifies this class down to two methods. The first is a helper method named Subscribe. Subscribe's job is to subscribe to the clock's OnSecondChange delegate. The second method is the event handler TimeHasChanged:

public class DisplayClock
{
    public void Subscribe(Clock theClock)
    {
        theClock.OnSecondChange +=
            new Clock.SecondChangeHandler(TimeHasChanged);
    }

    public void TimeHasChanged(
        object theClock, TimeInfoEventArgs ti)
    {
            Console.WriteLine("Current Time: {0}:{1}:{2}",
                ti.hour.ToString(), 
                ti.minute.ToString(), 
                ti.second.ToString());
   }
}

When the first method, Subscribe, is invoked, it creates a new SecondChangeHandler delegate, passing in its event handler method TimeHasChanged. It then registers that delegate with the OnSecondChange event of Clock.

You will create a second class that will also respond to this event: LogCurrentTime. This class would normally log the event to a file, but for our demonstration purposes, it will log to the standard console:

public class LogCurrentTime
{
    public void Subscribe(Clock theClock)
    {
        theClock.OnSecondChange +=
            new Clock.SecondChangeHandler(WriteLogEntry);
    }

    // this method should write to a file
    // we write to the console to see the effect
    // this object keeps no state
    public void WriteLogEntry(
        object theClock, TimeInfoEventArgs ti)
    {
        Console.WriteLine("Logging to file: {0}:{1}:{2}",
            ti.hour.ToString(), 
            ti.minute.ToString(), 
            ti.second.ToString());
    }
}

Although in this example these two classes are very similar, in a production program, any number of disparate classes might subscribe to an event.

All that remains is to create a Clock class, create the DisplayClock class, and tell it to subscribe to the event. You then will create a LogCurrentTime class and tell it to subscribe as well. Finally, you'll tell the Clock to run. All of this is shown in the following complete example:

Implementing Events with Delegates

namespace Programming_CSharp
{
  using System;
  using System.Threading;

  // a class to hold the information about the event
  // in this case it will hold only information 
  // available in the clock class, but could hold
  // additional state information 
  public class TimeInfoEventArgs : EventArgs
  {
    public TimeInfoEventArgs(int hour, int minute, int second)
    {
      this.hour = hour;
      this.minute = minute;
      this.second = second;
    }
    public readonly int hour;
    public readonly int minute;
    public readonly int second;
  }

  // our subject -- it is this class that other classes
  // will observe. This class publishes one delegate: 
  // OnSecondChange.
  public class Clock
  {
    // the delegate the subscribers must implement
    public delegate void SecondChangeHandler
      (
         object clock, 
         TimeInfoEventArgs timeInformation
      );

    // an instance of the delegate 
    public SecondChangeHandler OnSecondChange;

    // set the clock running
    // it will raise an event for each new second
    public void Run()
    {
      
      for(;;)
      {
        // sleep 10 milliseconds
        Thread.Sleep(10);
                
        // get the current time
        System.DateTime dt = System.DateTime.Now;

        // if the second has changed
        // notify the subscribers
        if (dt.Second != second)
        {
          // create the TimeInfoEventArgs object
          // to pass to the subscriber
          TimeInfoEventArgs timeInformation = 
            new TimeInfoEventArgs(
            dt.Hour,dt.Minute,dt.Second);

          // if anyone has subscribed, notify them
          if (OnSecondChange != null)
          {
            OnSecondChange(
              this,timeInformation);
          }
        }

        // update the state
        this.second = dt.Second;
        this.minute = dt.Minute;
        this.hour = dt.Hour;

      }
    }
    private int hour;
    private int minute;
    private int second;
  }

  // an observer. DisplayClock subscribes to the 
  // clock's events. The job of DisplayClock is 
  // to display the current time 
  public class DisplayClock
  {
    // given a clock, subscribe to 
    // its SecondChangeHandler event
    public void Subscribe(Clock theClock)
    {
      theClock.OnSecondChange +=
        new Clock.SecondChangeHandler(TimeHasChanged);
    }

    // the method that implements the 
    // delegated functionality
    public void TimeHasChanged(
      object theClock, TimeInfoEventArgs ti)
    {
      Console.WriteLine("Current Time: {0}:{1}:{2}",
        ti.hour.ToString(), 
        ti.minute.ToString(), 
        ti.second.ToString());
    }
  }

  // a second subscriber whose job is to write to a file
  public class LogCurrentTime
  {
    public void Subscribe(Clock theClock)
    {
      theClock.OnSecondChange +=
        new Clock.SecondChangeHandler(WriteLogEntry);
    }

    // this method should write to a file
    // we write to the console to see the effect
    // this object keeps no state
    public void WriteLogEntry(
      object theClock, TimeInfoEventArgs ti)
    {
      Console.WriteLine("Logging to file: {0}:{1}:{2}",
        ti.hour.ToString(), 
        ti.minute.ToString(), 
        ti.second.ToString());
    }
  }

  public class Test
  {
    public static void Main()
    {
      // create a new clock 
      Clock theClock = new Clock();

      // create the display and tell it to
      // subscribe to the clock just created
      DisplayClock dc = new DisplayClock();
      dc.Subscribe(theClock);

      // create a Log object and tell it
      // to subscribe to the clock 
      LogCurrentTime lct = new LogCurrentTime();
      lct.Subscribe(theClock);

      // Get the clock started
      theClock.Run();
    }
  }
}

Output:
Current Time: 14:53:56
Logging to file: 14:53:56
Current Time: 14:53:57
Logging to file: 14:53:57
Current Time: 14:53:58
Logging to file: 14:53:58
Current Time: 14:53:59
Logging to file: 14:53:59
Current Time: 14:54:0
Logging to file: 14:54:0

The net effect of this code is to create two classes, DisplayClock and LogCurrentTime, both of which subscribe to a third class' event (Clock.OnSecondChange).

OnSecondChange is a delegate. It starts out set to null. When the observer classes wish to be notified, they create an instance of the delegate and then add these delegates to OnSecondChange. For example, in DisplayClock's Subscribe method, you see this line of code:

theClock.OnSecondChange +=
  new Clock.SecondChangeHandler(TimeHasChanged);

It turns out that the LogCurrentTime class also wants to be notified. In its Subscribe method is very similar code:

public void Subscribe(Clock theClock)
{
  theClock.OnSecondChange +=
    new Clock.SecondChangeHandler(WriteLogEntry);
}

Problems with Delegates and Events

There is a problem with this example, however. What if the LogCurrentTime class was not so considerate, and it used the assignment operator (=) rather than the subscribe operator (+=)?

public void Subscribe(Clock theClock)
{
  theClock.OnSecondChange =
    new Clock.SecondChangeHandler(WriteLogEntry);
}

If you make that one tiny change to the example, you'll find that the Logger method is called but the DisplayClock method is not called. The assignment operator replaced the delegate held in the OnSecondChange multi-cast delegate. Not good.

A second problem is that other methods can call SecondChangeHandler directly. For example, you might add the following code to the Main() method of your Test class:

Console.WriteLine("Calling the method directly!");
System.DateTime dt = System.DateTime.Now.AddHours(2);

TimeInfoEventArgs timeInformation = 
  new TimeInfoEventArgs(
  dt.Hour,dt.Minute,dt.Second);

theClock.OnSecondChange(theClock, timeInformation);

Here, Main() has created its own TimeInfoEventArgs object and invoked OnSecondChange directly. This runs fine, even though it is not what the designer of the Clock class intended. Here is the output:

Calling the method directly!
Current Time: 18:36:7
Logging to file: 18:36:7
Current Time: 16:36:7
Logging to file: 16:36:7

The problem is that the designer of the Clock class intended the methods encapsulated by the delegate to be invoked only when the event is fired. Here Main() has gone around through the back door, and invoked those methods itself. What is more, it has passed in bogus data -- passing in a time construct set to two hours into the future!

How can you, as the designer of the Clock class, ensure that no one calls the delegated method directly? You can make the delegate private, but then it won't be possible for clients to register with your delegate at all! What is needed is a way to say "This delegate is designed for event handling: you may subscribe and unsubscribe, but you may not invoke it directly."

The solution to this dilemma is to use the event keyword. The event keyword indicates to the compiler that the delegate can only be invoked by the defining class, and that other classes can only subscribe to, and unsubscribe from, the delegate using the appropriate += and -= operators, respectively.

To fix your program, change your definition of OnSecondChange from

public SecondChangeHandler OnSecondChange; 

to the following:

public event SecondChangeHandler OnSecondChange;

Adding the event keyword fixes both problems. Classes can no longer attempt to subscribe to the event using the assignment operator (=), as you did above; nor can they invoke the event directly, as was done in Main in the example above. Either of these attempts will now generate a compile error:

The event 'Programming_CSharp.Clock.OnSecondChange' can only appear on the 
left hand side of += or -= (except when used from within the type 
'Programming_CSharp.Clock')

There are two ways of looking at OnSecondChange, now that you've modified it. In one sense, it is simply a delegate instance to which you've restricted access using the keyword event. In another, more important sense, OnSecondChange is an event, implemented by a delegate of type SecondChangeHandler. These two statements mean the same thing, but the latter is a more object-oriented way of looking at it, and better reflects the intent of this keyword: to create an event that your object can raise, and to which other objects can respond.

The complete source, modified to use the event, rather than the unrestricted delegate, is shown in the following modified example:

Using the Keyword event

namespace Programming_CSharp
{
  using System;
  using System.Threading;

  // a class to hold the information about the event
  // in this case it will hold only information 
  // available in the clock class, but could hold
  // additional state information 
  public class TimeInfoEventArgs : EventArgs
  {
    public TimeInfoEventArgs(int hour, int minute, int second)
    {
      this.hour = hour;
      this.minute = minute;
      this.second = second;
    }
    public readonly int hour;
    public readonly int minute;
    public readonly int second;
  }

  // our subject -- it is this class that other classes
  // will observe. This class publishes one event: 
  // OnSecondChange. The observers subscribe to that event
  public class Clock
  {
    // the delegate the subscribers must implement
    public delegate void SecondChangeHandler
      (
      object clock, 
      TimeInfoEventArgs timeInformation
      );

    // the keyword event controls access to the delegate
    public event SecondChangeHandler OnSecondChange;

    // set the clock running
    // it will raise an event for each new second
    public void Run()
    {
      
      for(;;)
      {
        // sleep 10 milliseconds
        Thread.Sleep(10);
                
        // get the current time
        System.DateTime dt = System.DateTime.Now;

        // if the second has changed
        // notify the subscribers
        if (dt.Second != second)
        {
          // create the TimeInfoEventArgs object
          // to pass to the subscriber
          TimeInfoEventArgs timeInformation = 
            new TimeInfoEventArgs(
            dt.Hour,dt.Minute,dt.Second);

          // if anyone has subscribed, notify them
          if (OnSecondChange != null)
          {
            OnSecondChange(
              this,timeInformation);
          }
        }

        // update the state
        this.second = dt.Second;
        this.minute = dt.Minute;
        this.hour = dt.Hour;

      }
    }
    private int hour;
    private int minute;
    private int second;
  }

  // an observer. DisplayClock subscribes to the 
  // clock's events. The job of DisplayClock is 
  // to display the current time 
  public class DisplayClock
  {
    // given a clock, subscribe to 
    // its SecondChangeHandler event
    public void Subscribe(Clock theClock)
    {
      theClock.OnSecondChange +=
        new Clock.SecondChangeHandler(TimeHasChanged);
    }

    // the method that implements the 
    // delegated functionality
    public void TimeHasChanged(
      object theClock, TimeInfoEventArgs ti)
    {
      Console.WriteLine("Current Time: {0}:{1}:{2}",
        ti.hour.ToString(), 
        ti.minute.ToString(), 
        ti.second.ToString());
    }
  }

  // a second subscriber whose job is to write to a file
  public class LogCurrentTime
  {
    public void Subscribe(Clock theClock)
    {
      theClock.OnSecondChange +=
        new Clock.SecondChangeHandler(WriteLogEntry);
    }

    // this method should write to a file
    // we write to the console to see the effect
    // this object keeps no state
    public void WriteLogEntry(
      object theClock, TimeInfoEventArgs ti)
    {
      Console.WriteLine("Logging to file: {0}:{1}:{2}",
        ti.hour.ToString(), 
        ti.minute.ToString(), 
        ti.second.ToString());
    }
  }

  public class Test
  {
    public static void Main()
    {
      // create a new clock 
      Clock theClock = new Clock();

      // create the display and tell it to
      // subscribe to the clock just created
      DisplayClock dc = new DisplayClock();
      dc.Subscribe(theClock);

      // create a Log object and tell it
      // to subscribe to the clock 
      LogCurrentTime lct = new LogCurrentTime();
      lct.Subscribe(theClock);


      // Get the clock started
      theClock.Run();
    }
  }
}

You can see that the event keyword serves to modify how the delegate is used, to be consistent with the semantics of event handling. There is much more to the use of delegates, and this topic is covered in depth in Chapter 12 of Programming C#, 3rd Edition, from which this article is adapted.


O'Reilly & Associates recently (in May 2003) released Programming C#, 3rd Edition.

Jesse Liberty is a senior program manager for Microsoft Silverlight where he is responsible for the creation of tutorials, videos and other content to facilitate the learning and use of Silverlight. Jesse is well known in the industry in part because of his many bestselling books, including O'Reilly Media's Programming .NET 3.5, Programming C# 3.0, Learning ASP.NET with AJAX and the soon to be published Programming Silverlight.


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