Chapter 8. Interfaces

An interface is a contract that guarantees to a client how a class or struct will behave. When a class (or struct) 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, Chapter 12 for information about events, and 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.

When you define an interface, you may define methods, properties, indexers, and/or events that will be implemented by the class that implements the interface.

Interfaces are often compared to abstract classes. 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.

For the rest of this chapter, wherever you see the word class, assume the text applies equally to structs, unless noted otherwise.

When a class implements an interface, it must implement all the parts of that interface (methods, properties, etc.); in effect, the class says "I agree to fulfill the contract defined by this interface."

Java programmers take note: C# doesn't support the use of constant fields (member constants) in interfaces. The closest analog is the use of enumerated constants (enums).

You will remember from Chapter 5 that inheriting from an abstract class implements the is-a relationship. Implementing an interface, on the other hand, defines a different relationship that we've not seen until now, called (not surprisingly) 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).

Mix-ins

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.