C# Cookbook

Chapter 7. Delegates and Events

Delegates contain all that is needed to allow a method, with a specific signature and return type, to be invoked by your code. A delegate can be used similarly to an object; for example, it can be passed to methods and stored in a data structure. A delegate is used when, at design time, you do not know which method you need to call and the information to determine this is available only at runtime.

Another scenario is when the code calling a method is being developed independently of the code that will supply the method to be called. The classic example is a Windows Forms control. If you design a control, you are unlikely to know what method should be called when the application raises an event, so you must use a delegate. However, when others use your control, they will typically decide at design time which method to call. (For example, it’s common to connect a Button’s click handler to a delegate at design time.)

This chapter’s recipes make use of delegates and events. These recipes cover handling each method invoked in a multicast delegate individually, synchronous delegate invocation versus asynchronous delegate invocation, and enhancing an existing class with events, among other topics. If you are not familiar with delegates and events, you should read the MSDN documentation on these topics. There are also good tutorials and example code showing you how to set up and use delegates and events.

7.1. Controlling when and if a Delegate Fires Within ...