csharp OOP's question

Question

Why should I use interface when an abstract class with non implemented methods behaves same as interface?

If below are the differences then there are some general question associated with them..Can some one please help me understand the existence  of interface..

 

1.Multiple inheritance was supported by interface..

In this case if we have single interface(inherited by a class) then that can be replaced by an abstract class without method implementations?

 

2.Abstract class supports default implementation of its methods..

In this case if I don't have any default implementation can I go for interface or an abstract class?