C# .NET - what is difference between abstract class and interface class

Abstract Class..
1.It cannot defines all the methods
2.It has subclass.
3.Here, Subclass is useless
4.A class can be extend an abstract class

Interface..
1.It defines all the methods
2.It must have implementations by other classes, But there will be no use of that.
3.Only an interface can extend another interface.

For More details..

http://kyapoocha.com/c-sharp-interview-questions/what%E2%80%99s-the-difference-between-an-interface-and-abstract-class-5/

http://www.dotnetuncle.com/Difference/4_abstract_class_interface.aspx

http://forums.msdn.microsoft.com/en-US/csharplanguage/thread/8ad621b8-a915-4d7e-89c3-5dbbc47202fd/

Best Luck!!!!!!!!!!!!!!!!!!!!!!!
Sujit.

1) Abstract class defines few or none of the methods, but 
interface defines all the methods..
2) Abstract classes should have subclasses else that will 
be useless..
Interfaces must have implementations by other classes else 
that will be useless
3)only an interface can extend another interface, but any 
class can extend an abstract class..
4)All variable in interfaces are final by default

5) Accessibility modifier(Public/Private/internal) is 
allowed for abstract class.
Interface doesn't allow accessibility modifier
6) class can inherit only one abstract class
class can implement more than one interface.

Interface: 1)In case if interface it is one contract.The 
functions which we define functions we have to make 
implements in body.
            2)It can come in inheritance chain
Abstract Class:1) In case of Abstract class we have to use 
keyword that is"Mustinherit".
               2)In that this is not come in inheritance 
chain

An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, 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. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards

Please check here for detailes explaination on the differences including sample code..

http://www.codeproject.com/KB/cs/abstractsvsinterfaces.aspx

Abstract Class:

1)   We can not instantiate the abstract class.

2)   Abstract class works like a base class.

3)   Abstract class must be inherited. (It comes in inheritance chain.)

4)   Abstract class can have abstract methods and discrete methods.

5)   Abstract methods must be implemented in derived class.

Interface:

1)   We must write prototype (methods declaration) in the interface.

2)   We need to implement the interface we can not inherit interface.

3)   Interface can be implemented to many classes.

4)   We can achieve multiple inheritance (technique) with the help of interfaces.

You cannot create an object of abstract class , but can make derivations of this.
An abstract class can contain either abstract methods or non abstract methods. Abstract members do not have any implementation in the abstract class, but the same has to be provided in its derived class.

An abstract class can have abstract members as well non abstract members. But in an interface all the members are implicitly abstract and all the members of the interface must override to its derived class.

Defining an abstract class with all the abstract members is similar to defining an interface. i.e we can say an interface is an abstract class with all the abstract members

Interfaces are essentially having all method prototypes no definition but Abstract class can contain method definations also.

In short Interface is a abstract class having all methods abstract.

Both abstract classes and interfaces are used when there is a difference in behaviour among the sub-types extending the abstract class or implementing the interface.

When the sub-types behaviour is totally different then you use an interface, when the sub-types behaviour is partially common and different with respect to the supertype an abstract class is used. In an abstract class the partially common behaviour is given a concrete implementation. Since there is no common behaviour between an interface and a sub-type an interface does not have an implementation for any of its behaviour.

If you create a abstract class writing the abstract keyword in the declaration part then You only can inherit the class. You can not create an instance of this abstract class but can inherit the class and with creating the instance of the derived class you can access the method of the abstract class.

If you use a virtual keyword in a method then you can override this method in the subclass if you wish..

If you create a abstract method then you must override this method in the subclass other wise it shows error in the program.

Refer msdn difference for more info..
http://sadi02.wordpress.com/2008/05/08/what-is-difference-in-an-abstract-class-and-an-interface/

http://msdn.microsoft.com/msdnmag/issues/03/12/CQA/