Learning C# 3.0

What Interfaces Are

An interface is a contract. When you design an interface, you’re saying, “If you want to provide this capability, you must implement these methods, provide these properties and indexers, and support these events.” The implementer of the interface agrees to the contract and implements the required elements.

Tip

You saw methods and properties in Chapter 8. We’ll discuss indexers in Chapter 14 and events in Chapter 17. We promise you don’t need to know about them for this chapter.

When you specify interfaces, it is easy to get confused about who is responsible for what. There are three concepts to keep clear:

The interface

This is the contract. By convention, interface names begin with a capital I, so your interface might have a name such as IPrintable. The IPrintable interface might require, among other things, a Print( ) method. This states that any class that wants to implement IPrintable must implement a Print( ) method, but it does not specify how that method works internally. That is up to the designer of the implementing class.

The implementing class

This is the class that agrees to the contract described by the interface. For example, Document might be a class that implements IPrintable and thus implements the Print( ) method in whatever way the designer of the Document class thinks is appropriate.

The client class

The client calls methods on the implementing class. For example, you might have an Editor class that has an array of IPrintable objects (every object ...