Events and Callbacks
Note
This content is reprinted by permission of Pearson Education, Inc. from Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries, 2nd Edition. That edition was published in 2008, and the book has since been fully revised in the third edition. Some of the information on this page may be out-of-date.
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.
❌ 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 run time 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.