4.4 Events

Event handling is essentially a process in which one object can notify other objects that an event has occurred. This process is largely encapsulated by multicast delegates, which have this ability built in.

4.4.1 Defining a Delegate for an Event

The .NET Framework provides many event-handling delegates, but you can write your own. For example:

delegate void MoveEventHandler(object source, MoveEventArgs e);

By convention, the delegate's first parameter denotes the source of the event, and the delegate's second parameter derives from System.EventArgs and contains data about the event.

4.4.2 Storing Data for an Event with EventArgs

The EventArgs class may be derived from to include information relevant to a particular event:

public class MoveEventArgs : EventArgs {
  public int newPosition;
  public bool cancel;
  public MoveEventArgs(int newPosition) {
    this.newPosition = newPosition;
  }
}

4.4.3 Declaring and Firing an Event

A class or struct can declare an event by applying the event modifier to a delegate field. In this example, the slider class has a Position property that fires a Move event whenever its Position changes:

class Slider {
  int position;
  public event MoveEventHandler Move;
  public int Position {
    get { return position; }
    set {
      if (position != value) { // if position changed
        if (Move != null) { // if invocation list not empty
          MoveEventArgs args = new MoveEventArgs(value);
          Move(this, args); // fire event
          if (args.cancel)
            return;
        }
        position = value;
      }
    }  
  }
}

The event keyword promotes encapsulation by ensuring that only the += and -= operations can be performed on the delegate. Other classes may act on the event, but only the Slider can invoke the delegate (fire the event) or clear the delegate's invocation list.

4.4.4 Acting on an Event with an Event Handler

We are able to act on an event by adding an event handler to it. An event handler is a delegate that wraps the method we want invoked when the event is fired.

In this example, we want our Form to act on changes made to a Slider's Position. This is done by creating a MoveEventHandler delegate that wraps our event-handling method (the slider_Move method). This delegate is added to the Move event's existing list of MoveEventHandlers (which is initially empty). Changing the position on the slider fires the Move event, which invokes our slider_Move method:

using System;
class Form {
  static void Main( ) {
    Slider slider = new Slider( );
    // register with the Move event
    slider.Move += new MoveEventHandler(slider_Move);
    slider.Position = 20;
    slider.Position = 60;
  }
  static void slider_Move(object source, MoveEventArgs e) {
    if(e.newPosition < 50)
      Console.WriteLine("OK");
    else {
      e.cancel = true;
      Console.WriteLine("Can't go that high!");
    }
  }
}

Typically, the Slider class is extended so that it fires the Move event whenever its Position is changed by a mouse movement, keypress, etc.

4.4.5 Event Accessors

attributes? unsafe? access-modifier?

[
  [[sealed | abstract]? override] |
  new? [virtual | static]?
]?
event delegate type event-property accessor-name
{
  attributes? add statement-block
  attributes? remove statement-block
}

Abstract accessors don't specify an implementation, so they replace an add/remove block with a semicolon.

Similar to the way properties provide controlled access to fields, event accessors provide controlled access to an event. Consider the following field declaration:

public event MoveEventHandler Move;

Except for the underscore prefix added to the field (to avoid a name collision), this is semantically identical to:

private MoveEventHandler _Move;
public event MoveEventHandler Move {
  add {
    _Move += value;
  }
  remove {
    _Move -= value;
  }
}

The ability to specify a custom implementation of add and remove handlers for an event allows a class to proxy an event generated by another class, thus acting as a relay for an event rather than the generator of that event. Another advantage of this technique is to eliminate the need to store a delegate as a field, which can be costly in terms of storage space. For instance, a class with 100 event fields would store 100 delegate fields, even though maybe only 4 of those events are actually assigned. Instead, you can store these delegates in a dictionary, and add and remove the delegates from that dictionary (assuming the dictionary holding 4 elements uses less storage space than 100 delegate references).

The add and remove parts of an event are compiled to add_XXX and remove_XXX methods.