An interface is a contract that guarantees to a client how a class or struct will behave. When a class implements an interface, it tells any potential client "I guarantee I'll support the methods, properties, events, and indexers of the named interface." (See Chapter 4 for information about methods and properties, see Chapter 12 for info about events, and see Chapter 9 for coverage of indexers.)
An interface offers an alternative to an abstract class for creating contracts among classes and their clients. These contracts are made manifest using the interface keyword, which declares a reference type that encapsulates the contract.
Syntactically, an interface is like a class that has only abstract methods. An abstract class serves as the base class for a family of derived classes, while interfaces are meant to be mixed in with other inheritance trees.
When a class implements an interface, it must implement all the methods of that interface; in effect the class says "I agree to fulfill the contract defined by this interface."
Inheriting from an abstract class implements the is-a relationship, introduced in Chapter 5. Implementing an interface defines a different relationship that we've not seen until now: the implements relationship. These two relationships are subtly different. A car is-a vehicle, but it might implement the CanBeBoughtWithABigLoan capability (as can a house, for example).
In Somerville, Massachusetts, there was, at one time, an ice cream parlor where you could have candies and other goodies "mixed in" with your chosen ice cream flavor. This seemed like a good metaphor to some of the object-oriented pioneers from nearby MIT who were working on the fortuitously named SCOOPS programming language. They appropriated the term "mix in" for classes that mixed in additional capabilities. C++ includes a number of mix-in classes as well. These mix-in or capability classes serve much the same role as interfaces do in C#.
In this chapter, you will learn how to create, implement, and use interfaces. You'll learn how to implement multiple interfaces, and how to combine and extend interfaces, as well as how to test whether a class has implemented an interface.