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 ...

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