Event accessors

Event declarations typically omit event-accessor-declarations, as in the Button example above. One situation for doing so involves the case in which the storage cost of one field per event is not acceptable. In such cases, a class can include event-accessor-declarations and use a private mechanism for storing the list of event handlers.

The event-accessor-declarations of an event specify the executable statements associated with adding and removing event handlers.

The accessor declarations consist of an add-accessor-declaration and a remove-accessor-declaration. Each accessor declaration consists of the token add or remove followed by a block. The block associated with an add-accessor-declaration specifies the statements to execute when an event handler is added, and the block associated with a remove-accessor-declaration specifies the statements to execute when an event handler is removed.

Each add-accessor-declaration and remove-accessor-declaration corresponds to a method with a single value parameter of the event type and a void return type. The implicit parameter of an event accessor is named value. When an event is used in an event assignment, the appropriate event accessor is used. Specifically, if the assignment operator is += then the add accessor is used, and if the assignment operator is -= then the remove accessor is used. In either case, the right-hand operand of the assignment operator is used as the argument to the event accessor. The block of an add-accessor-declaration or a remove-accessor-declaration must conform to the rules for void methods described in §10.6.10. In particular, return statements in such a block are not permitted to specify an expression.

Since an event accessor implicitly has a parameter named value, it is a compile-time error for a local variable or constant declared in an event accessor to have that name.

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.