Interface vs Base class

Question

When should I use an interface and when should I use a base class?

Should it always be an interface if I don\'t want to actually define a base implementation of the

Answer 1

Previous comments about using abstract classes for common implementation is definitely on the mark. One benefit I haven't seen mentioned yet is that the use of interfaces makes it much easier to implement mock objects for the purpose of unit testing. Defining IPet and PetBase as Jason Cohen described enables you to mock different data conditions easily, without the overhead of a physical database (until you decide it's time to test the real thing).