C# 3.0 Cookbook, 3rd Edition

Chapter 9. Delegates, Events, and Lambda Expressions

9.0. Introduction

A delegate is an object which represents a method and, optionally, the "this" object associated with that method. When the delegate is invoked, the corresponding method is invoked. 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 passed to methods, and a reference to it can be stored in a structure or class. 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 create a control, you are unlikely to know what method should be called in the application when the control raises an event, so you must provide a delegate to allow the application to hook up a handler to the event. When other developers use your control, they will typically decide when they are adding the control (through the designer or programmatically) and which method should be called to handle the event published by the control. (For example, it's common to connect a Button's click handler to a delegate at design time.)

Events are a specialized delegate type primarily used for message or notification passing. Events can only be invoked from the type they are published ...