Programming .NET Windows Applications

Publish and Subscribe

In .NET, controls publish a set of events to which other classes can subscribe. When the publishing class raises an event, all the subscribed classes are notified.

Tip

This design is similar to the Publish/Subscribe (Observer) Pattern described in the seminal work Design Patterns by Gamma, et al. (Addison Wesley). 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 event mechanism, the control says, "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.

Events and Delegates

Events are implemented with delegates. The publishing class defines a delegate that encapsulates a method that the subscribing classes implement. When the event is raised, the subscribing classes' methods (the event handlers) are invoked through the delegate.

Note

A delegate type defines the signature of methods that can be encapsulated by instances of that delegate type. A delegate can be marked as an event to restrict access to that delegate for use as an event handler.

For more information on the relationship between delegates and ...