20. Interfaces

An interface defines a contract. A class or struct that implements an
interface must adhere to its contract. An
interface may inherit from multiple base interfaces, and a class or struct
may implement multiple interfaces.
Interfaces can contain methods, properties, events, and indexers. The
interface itself does not provide
implementations for the members that it defines. The interface merely
specifies the members that must be
supplied by classes or interfaces that implement the interface.