Use of an abstract class and interface

Use an abstract class

·         When creating a class library which will be widely distributed or reused—especially to clients, use an abstract class in preference to an interface; because, it simplifies versioning. This is the practice used by the Microsoft team which developed the Base Class Library. (COM was designed around interfaces.)

·         Use an abstract class to define a common base class for a family of types.

·         Use an abstract class to provide default behavior.

·         Subclass only a base class in a hierarchy to which the class logically belongs.

Use an interface

·         When creating a standalone project which can be changed at will, use an interface in preference to an abstract class; because, it offers more design flexibility.

·         Use interfaces to introduce polymorphic behavior without subclassing and to model multiple inheritance—allowing a specific type to support numerous behaviors.

·         Use an interface to design a polymorphic hierarchy for value types.

·         Use an interface when an immutable contract is really intended.

·         A well-designed interface defines a very specific range of functionality. Split up interfaces that contain unrelated functionality.