Programming .NET Components, 2nd Edition

Delegate-Based Events

Before I describe .NET event support, here are a few terms. The object publishing the event is called the source or the publisher, and any party interested in the event is called a sink or a subscriber. The event notifications are in the form of the publisher calling methods on the subscribers. Publishing an event is also called firing an event. .NET offers native support for events by providing dedicated CLR types and base-class implementations. .NET defines a standard mechanism for source and sink connection setup and tear-down, a standard and concise way of firing events, and a ready-made implementation of the sink list.

.NET event support relies on delegates . Conceptually, a delegate is nothing more than a type-safe method reference—you can think of it as a type-safe C function pointer or a function object in C++. As the name implies, a delegate allows you to delegate the act of calling a method to somebody else. The delegate can call static or instance methods. Consider, for example, the delegate NumberChangedEventHandler, defined as:

    public delegate void NumberChangedEventHandler(int number);

This delegate can be used to call any method with a matching signature (a void return type and one int parameter). The name of the delegate, the names of the target methods, and the names of those methods’ parameters are of no importance. The only requirement is that the methods being called have the exact signature (i.e., the same types) that the delegate expects. ...