Question

Whenever i hear about interfaces i have the following doubt.

i have the following interface

interface Imammals
          {
           walk();
           eat();
           run();
          }

and i have two classes Human and Cat that implements this interface.

Anyway, the functionality of the methods are going to be different in both the Classes.

For Eg: walk(), the functionality differs as cat uses four legs and human uses two legs

Then, Why do i need to have a common interface which ONLY declares these methods? Is my design here faulty?

If the functionality of the methods are going to be same in both the classes, i could go for a class based inheritance where the parent implements the complete functionality and the child inherits and uses the parent class methods.

But here the interfaces help us just to consolidate the methods declarations or is there anything more inside?

EDIT: walking(), eating(), running() was changed to walk(), eat(), run() and mammals was changes to Imammals.

Answer 1

In your scenario, either type-inheritance or interface-implementation would work – but interface based abstraction allows types outside of your existing type model to provide the functionality. It could be a mock object, or it could be some kind of super killer robot, that can walk run and eat but isn’t really a mammal (so having it inherit from a Mammal class could be confusing or just impossible).

In particular, interfaces allow us to express this relationship neatly, while avoiding the subtle points from C# having single (type-)inheritance.