Use following guidelines for each of the abstractions:
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.
