Most of the time developers get confused about abstract class and interface. while interview most of the time interviewer asks the difference between abstract class and interface, we give answer that we read on internet and what we have understanding, but it gets messed up always.


Also when developing applications, we are not sure often that it is the scenario to use abstract class or create interface.

I am posting text from CLR via C# book where Jeffery Richter is trying to explain when should we use abstract class and when interface. Here is the text from book:

I often hear the question "Should i design a base type or interface ?" The answer is not always clear cut.

Here are some guidelines that might help you:

• IS - A VS CAN-DO relationship : A type can inherit only one implementation If the Derived type can't claim  IS-A relationship with the base type, don't use a base type, use an interface. interfaces imply a CAN-DO relationship. If the CAN-DO functionality appears to belong with various object types, use an interface. For example, a type can convert instances of itself to another type (IConvertible), a type can serialize an instance of itself (ISerializable), etc. Note that value types must be derived from System.ValueType, and therefore, they cannot be derived from an arbitrary base class. In this case, you must use a CAN-DO relationship and define an interface.
• Ease of Use: It’s generally easier for you as a developer to define a new type derived from a base type than to implement all of the methods of an interface. The base type can provide a lot of functionality, so the derived type probably needs only relatively small modifications to its behavior. If you supply an interface, the new type must implement all of the members.
• Consistent Implementation:No matter how well an interface contract is documented, it’s very unlikely that everyone will implement the contract 100 percent correctly. In fact, COM suffers from this very problem, which is why some COM objects work correctly only with Microsoft Word or with Windows Internet Explorer. By providing a base type with a good default implementation, you start off using a type that works and is well tested, you can then modify parts that need modification.
• Versioning:If you add a method to the base type, the derived type inherits the new method, you start off using a type that works, and the user’s source code doesn't even have to be recompiled. Adding a new member to an interface forces the inheritor of the interface to change its source code and recompile.