Abstract Class and Interface

Abstract Class:
     An abstract class is a special kind of class that cannot be instantiated. i.e. It only allows other class to inherit from but it cannot be instantiated.

    Advatage:
         Enforces certain hierarchies for all the subclasses. ie. It is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

Interface:
    An interface is not a class. It is an entity that is defined by the name interface. An interface has no implementation. It is only the signature or just definition of the methods without the body. It is a kind of contract that is used to define the hierarchies for all the subclasses or it defines specific set of methods and arguments. As c# does not support multiple inheritance, interface are used to implement multiple inheritance.

Purpose of Abstract class and Interface:
    When we create an interface, we basically create a set of methods without any implemention which must be overridden by the inherited class. Advantage is that, it provides a way for a class to be part of two classes: one from inheritance hirearchy and one from the interface.
    When we create an abstract class, we are creating a base class that might have one or more implemented methods and atleast one or more methods are left without implementing and declared abstract. The purpose of an abstract class is to provide a base class definition of how aset of derived class will work.

Interface and Abstract class - Comparison

Feature	Interface	Abstract Class
Multiple inheritance	A class may inherit several interfaces.	A class may inherit only one abstract class.
Default implementation	An interface cannot provide any code, just the signature.	An abstract class can provide complete, default code and/or just the details that have to be overridden.
Access Modfiers	An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public	An abstract class can contain access modifiers for the subs, functions, properties
Core VS Peripheral	Interfaces are used to define the peripheral abilities of a class. In other words both Human and Vehicle can inherit from a IMovable interface.	An abstract class defines the core identity of a class and there it is used for objects of the same type.
Homogeneity	If various implementations only share method signatures then it is better to use Interfaces.	If various implementations are of the same kind and use common behaviour or status then abstract class is better to use.
Speed	Requires more time to find the actual method in the corresponding classes.	Fast
Adding functionality (Versioning)	If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.	If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.
Fields and Constants	No fields can be defined in interfaces	An abstract class can have fields and constrants defined