Benefits of implementing an interface

Question

what are the benefits of implementing an interface in C# 3.5 ?

Answer 1

The way I understand it interfaces are most useful in these cases:

1. Cleaner division of labor among programmers. Lead programmer writes interface and junior programmer writes its implementation. That makes perfect sense to me. Lead programmer could write pseudocode instead of interface though.

2. Some specific situation, where you need 2 or more different implementations of the same class, for example interface animal and classes tiger and lion that use it. And even here it doesn't makes much sense, because lions and tigers share some things in common. Abstract class would be better, because if you use interface you have to write common functions in separate classes which leads to code duplication, which is bad.

3. You write a library and want it to be modifiable by users. So you write interface and its class implementation. User of your lib still has the possibility to write his own implementation class, which may use different technology/algorithm which achieves the same result, but maybe in a faster way for example. This is also the reason why we meet so many interfaces in libs we use, but rarely feel the need to write our own interfaces. Because we don't write libraries.