[Example: In the example
class Control: Component
{
// Unique keys for events
static readonly object mouseDownEventKey = new object();
static readonly object mouseUpEventKey = new object();
// Return event handler associated with key
protected Delegate GetEventHandler(object key) {...}
// Add event handler associated with key
protected void AddEventHandler(object key, Delegate handler) {...}
// Remove event handler associated with key
protected void RemoveEventHandler(object key, Delegate handler) {...}
// MouseDown event
public event MouseEventHandler MouseDown {
add { AddEventHandler(mouseDownEventKey, value); }
remove { RemoveEventHandler(mouseDownEventKey, value); }
}
// MouseUp event
public event MouseEventHandler MouseUp {
add { AddEventHandler(mouseUpEventKey, value); }
remove { RemoveEventHandler(mouseUpEventKey, value); }
}
// Invoke the MouseUp event
protected void OnMouseUp(MouseEventArgs args) {
MouseEventHandler handler;
handler = (MouseEventHandler)GetEventHandler(mouseUpEventKey);
if (handler != null)
handler(this, args);
}
}
the Control class implements an internal storage mechanism for events. The AddEventHandler method associates a delegate value with a key, the GetEventHandler method returns the delegate currently associated with a key, and the RemoveEventHandler method removes a delegate as an event handler for the specified event. Presumably, the underlying storage mechanism is designed such that there is no cost for associating a null delegate value with a key, and thus unhandled events consume no storage. end example]