Today somebody told me that interface implementation in C# is just "Can-Do" relationship, not "Is-A" relationship. This conflicts with my long-time believing in LSP(Liskov Substitution Principle). I always think that all inheritance should means "Is-A" relationship.
So, If interface implementation is just a "Can-Do" relationship. What if there is a interface "IHuman" and "IEngineer", and one class "Programmer" inherits from "IHuman" & "IEngineer"? Surely, a "Programmer" Is A "IHuman" and A "IEngineer".
If it is just "Can-Do" relationship, does it mean we cannot expect the "Programmer" instance behavior may be different between when treated as a IHuman and treated as IEngineer?
In my experience it doesn't really help that much to think of "is-a" and "can-do" relationships. You rapidly get into problems. It's an impedance mismatch between the real world and OO, basically. However much people actually talk about modeling the real world, you fundamentally need to understand what the relationships between types mean on the platform you're using.
Sometimes interfaces can be used as capabilities, and sometimes they can represent more of a normal "is-a" relationship. I wouldn't get too hung up about it - just make sure you understand what they can do and what they can't.