How to mock an object that extends a class and implements an interface?

What your design is really saying is that your class MyClass does not have a dependency on IFoo it has a dependency on a class instance derived from BaseFoo and implementing IFoo . In this case it is probably best that you introduce a unifying interface which you can then use as a dependency and mock.

How about declaring an abstract class (BaseFooFoo?) that derives from BaseFoo and implements IFoo. Then you could show BaseFooFoo to moq.

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