C# - Is adding systematically an interface a good practice?

Question

In the project I'm working on, I've noticed that for every entity class there is an interface. It seems that the original motivation was to only expose interfaces to other project/solutions.

I find this completely useless, and I don't see the point in creating an interface for every class. By the way, those classes don't have any methods just properties and they don't implement the same interface.

Am I wrong? Or is it a good practice?

Thx

Answer 1

I tend to create an interface for almost every class mainly because of unit testing - if you use dependency injection and want to unit test a class that depends on the class in question, than the standard way is to mock an instance of the class in question (using one of the mocking frameworks, e.g. Rhino-Mocks). However, practically it is only possible only for interfaces, not concrete implementations (yes, theoretically you can mock a concrete class, but there are many painful limitations).