Chapter 8. Interfaces

An interface is a contract that guarantees to a client how a class or structure 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 5 for information about methods and properties, see Chapter 12 for information about events, and see Chapter 9 for coverage of indexers.)

An interface offers an alternative to a MustInherit class (see Chapter 6) 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 MustInherit methods. A MustInherit 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 a MustInherit class implements the is-a relationship, introduced in Chapter 4. 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).

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