Interface and abstract clarification

I hope you will get what you want:

An interface defines a contract - something anything implementing that interface MUST do. It can NEVER specify how that something is done.

An abstract class is similar in that it also defines what must be done by a class implementing it, but it can have some implementation as well. In many cases, these implementations are considered as default implementations and can be overridden in derived classes. If derived classes don't ovverride it, than the abstract class's implementation is used. If you want something that can be used in all derived classes, you can put it in the base class.

Although they both provide a contract of sorts, they are different. A class can implement many intrerfaces, but can only have a single base class. Using an interface, when you publish it, it's pretty much sealed. Any change in the interface will require you to change every implementation of that interface. If you publish the interface to many developers, that's not really an option. Using an abstract base class, you can add some behaviour / data to the base class and if done properly, it won't need any changes in code for classes deriving from the base class. All existing code will exist to work. But this takes away the single base class a class can derive from and enforces the "evil" of inheritance.

Think of an interface in terms of behaviour and abstract base classes in terms of implementation.

Reference articles:
http://forums.asp.net/t/1523088.aspx
http://www.codeproject.com/Questions/43970/Real-world-examples-of-abstract-classes-and-interf.aspx

Read these articles :
Interface Live Examples
Abstract Class Live Examples

Thanks, i got to know the details of interface and abstract. however, I would like to know the real time projects usage apart from animals, vechile and general examples available on internet.

>>I would like to know the real time projects usage apart from animals, vechile and general examples available on internet.


Does it really help if I said XXX corporation uses both techniques in their app YYY  (which could involve vehicles unfortunately so any details would be rejected by your clause).  The main point is that both do have their uses.

I think this will help you for sure:

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

The above example use the employee class usage.

can you please help me with small expample here only