Events and Callbacks

Callbacks are extensibility points that allow a framework to call back into user code through a delegate. These delegates are usually passed to the framework through a parameter of a method.

Events are a special case of callbacks that supports convenient and consistent syntax for supplying the delegate (an event handler). In addition, Visual Studio’s statement completion and designers provide help in using event-based APIs. (See Event Design.)

✓ CONSIDER using callbacks to allow users to provide custom code to be executed by the framework.

✓ CONSIDER using events to allow users to customize the behavior of a framework without the need for understanding object-oriented design.

✓ DO prefer events over plain callbacks, because they are more familiar to a broader range of developers and are integrated with Visual Studio statement completion.

X AVOID using callbacks in performance-sensitive APIs.

✓ DO use the new Func<...>, Action<...>, or Expression<...> types instead of custom delegates, when defining APIs with callbacks.

Func<...> and Action<...> represent generic delegates. Expression<...> represents function definitions that can be compiled and subsequently invoked at runtime but can also be serialized and passed to remote processes.

✓ DO measure and understand performance implications of using Expression<...>, instead of using Func<...> and Action<...> delegates.

Expression<...> types are in most cases logically equivalent to Func<...> and Action<...> delegates. The main difference between them is that the delegates are intended to be used in local process scenarios; expressions are intended for cases where it’s beneficial and possible to evaluate the expression in a remote process or machine.

✓ DO understand that by calling a delegate, you are executing arbitrary code and that could have security, correctness, and compatibility repercussions.

Portions © 2005, 2009 Microsoft Corporation. All rights reserved.

Reprinted by permission of Pearson Education, Inc. from Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries, 2nd Edition by Krzysztof Cwalina and Brad Abrams, published Oct 22, 2008 by Addison-Wesley Professional as part of the Microsoft Windows Development Series.

See Also

Designing for Extensibility
Framework Design Guidelines