An abstract class is a special kind of class that cannot be instantiated. An abstract class is only to be sub-classed .It only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. It is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.
An interface is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature . It is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. Since C# doesn’t support multiple inheritance, interfaces are used to implement multiple inheritance.
The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class.
Playing with Abstract and interface:
When we create an interface, we are basically creating a set of methods without any implementation that must be overridden by the implemented classes. The advantage is that it provides a way for a class to be a part of two classes: one from inheritance hierarchy and one from the interface.
When we create an abstract class, we are creating a base class that might have one or more completed methods but at least one or more methods are left uncompleted and declared abstract.
If all the methods of an abstract class are uncompleted then it is same as an interface.
The purpose of an abstract class is to provide a base class definition for how a set of derived classes will work and then allow the programmers to fill the implementation in the derived classes.