OOPS

Hi,

First check what is abstract class and interface.
http://www.codeproject.com/Articles/6118/All-about-abstract-classes
http://www.codeproject.com/Articles/18743/Interfaces-in-C-For-Beginners

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.

Abstract classes can include implementations of methods which can be over-ridden in derived classes. Interfaces cannot include implementations. Base on that definition, we can simply say that use abstract classes when you want to have a base class that has default implementations and overridable methods. And use interface to define a protocol of behavior that can be implemented by any class anywhere in the class hierarchy.

An abstract class can't be instantiated directly.  Therefore, to use its functionality it must be inherited by another class which defines the implementation of any abstract members.  In this way it is similar to an interface.  The difference between an interface and an abstract class is that an interface has zero implementation details.  All members of an interface are abstract and must be implemented.  An abstract class can have a combination of abstract and concrete members.

also  read  this link


 http://www.codeproject.com/Questions/382073/Where-to-Use-Abstact-Class-and-Where-to-Use-Interf