Why I need Interface

[Umeshnath]

My query is why I need an Interface, since my abstract class does the same
functionality

Interface
No implementation of methods - ------- This can achieved by abstract class
No instance creation just inheriting ----- This can achieved by abstract class

I know using interface we can go multiple inheritance
I am looking in such way the in terms of OOPS in C# what is the significance

Also I know there are certain places (WWF, WCF) only interface allows as
contracts, those things are MS design part.
I am looking why I need interface, if Abstract class does every thing

Thanks & Regards
Umeshnath