Abstract(patially implemented) class can be used as a 
common base class for an application it has all property of 
class except it cannot be instantiated.it provide default 
behavior.

interface is mearly a specification,nothing implemented for 
any standalone project which can be changed at will its 
more design flexible and it can be utilized to model 
multipleinheritance.

Hi...
When u want to inherit the properties of base class only
(not other resources) go for Abstract class.
When u want to inherit the properties of base class and 
other resources go for Interfaces .

abstract class is used if you are willing to restrict the 
creation of an instance.
interface is used if you are willing to write different 
functionalities in different implementations.

In Interfate you are restricting a user to impement each 
and every method of interface so if you have a requirement 
which is  constantly changing so don't prefere interface.

While in Abstract Class user don't need to implement each 
and every method which is not required or marked with 
mustoverride.


And by using interface you can achive multiple inheritance 
in C# but with Abstact class it's can't be achive

Abstract provides both methods with and without body. 
Adding new methods in abstract class in middle of 
development life cycle won't break existing sub classes 
unless these methods are declared as mustoverride. If there 
are frequent addition of new methods properties and so on. 
one should use abstract..

Whereas on the other hand interface can fill gap of 
multiple inheritance. One can
implement from more than one interface. Not same with 
Abstract. One can inherit
from only one abstract class.