Answer Posted / ashis

Interface:

–> If your child classes should all implement a certain
group of methods/functionalities but each of the child
classes is free to provide its own implementation then use
interfaces.

For e.g. if you are implementing a class hierarchy for
vehicles implement an interface called Vehicle which has
properties like Colour MaxSpeed etc. and methods like
Drive(). All child classes like Car Scooter AirPlane
SolarCar etc. should derive from this base interface but
provide a seperate implementation of the methods and
properties exposed by Vehicle.

–> If you want your child classes to implement multiple
unrelated functionalities in short multiple inheritance use
interfaces.

For e.g. if you are implementing a class called SpaceShip
that has to have functionalities from a Vehicle as well as
that from a UFO then make both Vehicle and UFO as interfaces
and then create a class SpaceShip that implements both
Vehicle and UFO .

Abstract Classes

–> When you have a requirement where your base class should
provide default implementation of certain methods whereas
other methods should be open to being overridden by child
classes use abstract classes.

For e.g. again take the example of the Vehicle class above.
If we want all classes deriving from Vehicle to implement
the Drive() method in a fixed way whereas the other methods
can be overridden by child classes. In such a scenario we
implement the Vehicle class as an abstract class with an
implementation of Drive while leave the other methods /
properties as abstract so they could be overridden by child
classes.

–> The purpose of an abstract class is to provide a common
definition of a base class that multiple derived classes can
share.

For example a class library may define an abstract class
that is used as a parameter to many of its functions and
require programmers using that library to provide their own
implementation of the class by creating a derived class.
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.

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