Interface over pure abstract class

In C#, a class can only derive from one base class - which can be abstract or concrete. If none is specified, the class derives directly from Object. MS reasoned that adding multiple inheritance added too much complexity to C# while providing too little benefit.
This gives us a problem which is solved via Interfaces - a class can also implement as many Interfaces as it wants (using the same syntax as derivation).
There is some "blurring" going on between abstract classes and interfaces in the latest C# version, now that Interfaces can provide default method implementations, but it's still not quite the same as "full derivation" which is still only possible with a single class.

Quote:

Yes. I knew that. Consider, I have only one interface with few methods. In that case, I can always create a pure abstract class and get my job done. Isn't it?
Then there must be some other reason too why to use interface and not pure abstract class or vice-versa.
It could be related to builds or implementation or security whatever.
I am thinking from all aspects actually.
Please let me know if you also can find out some other points.
Thanks a lot.

There are no other points - it is purely about Single Inheritance.
Think about it: you have a class which has to inherit from SqlDataReader for example. That means the class definition must be:

public MyClass : SqlDataReader
   {
   ...
   }

Suppose you also want that class to be usable in foreach loops: that means it must implement the IEnumerator interface^*. Since C# does not allow multiple inheritance,without interfaces you would have to choose between deriving from SqlDataReader and IEnumerator. But, because you can implement as many Interfaces as you want, you can have your cake and eat it:

public MyClass : SqlDataReader, IEnumerator
   {
   ...
   }

Without having the extra complexity (and there is a load of that) that multiple inheritance requires.


* Over simplification, I know - just go with it.