At times a designer does not want to create a new type. Rather, the designer wants to describe a set of behaviors that any number of types might implement. For example, a designer might want to describe what it means to be storable (i.e., capable of being written to disk or to a database) or printable.
Such a description is called an interface. An interface is a contract: the designer of the interface says “if you want to provide this capability, you must implement these methods.” The implementer of the interface agrees to the contract and implements the required methods.
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.” The interface details the return type from each method and the parameters to the methods.
When specifying interfaces, it is easy to get confused about who is responsible for what. There are three concepts to keep clear:
This is the contract. By convention, interface names begin with a capital I; thus, your interface might have a name like IPrintable. The IPrintable interface might describe a Print( ) method.
This is the class that agrees to the contract described by the interface. For example, Document might be a class that implements IPrintable ...