How do you unit test private methods?

Question

I\'m building a class library that will have some public & private methods. I want to be able to unit test the private methods (mostly while developing, but also it coul

Answer 1

I think a more fundamental question should be asked is that why are you trying to test the private method in the first place. That is a code smell that you're trying to test the private method through that class' public interface whereas that method is private for a reason as it's an implementation detail. One should only be concerned with the behaviour of the public interface not on how it's implemented under the covers.

If I want to test the behaviour of the private method, by using common refactorings, I can extract its code into another class (maybe with package level visibility so ensure it's not part of a public API). I can then test its behaviour in isolation.

The product of the refactoring means that private method is now a separate class that has become a collaborator to the original class. Its behaviour will have become well understood via its own unit tests.

I can then mock its behaviour when I try to test the original class so that I can then concentrate on test the behaviour of that class' public interface rather than having to test a combinatorial explosion of the public interface and the behaviour of all its private methods.

I see this analogous to driving a car. When I drive a car I don't drive with the bonnet up so I can see that the engine is working. I rely on the interface the car provides, namely the rev counter and the speedometer to know the engine is working. I rely on the fact that the car actually moves when I press the gas pedal. If I want to test the engine I can do checks on that in isolation. :D

Of course testing private methods directly may be a last resort if you have a legacy application but I would prefer that legacy code is refactored to enable better testing. Michael Feathers has written a great book on this very subject. http://www.amazon.co.uk/Working-Effectively-Legacy-Robert-Martin/dp/0131177052