Do not test private methods

@guy:

Microsoft's opinion is in favor of testing private methods. In fact, they seem to encourage it with, like you said, Private Accessors and various How To articles on MSDN.

I wouldn't say that this is just my opinion, it is generally a common-held opinion in the unit testing and, specifically, TDD circles. I won't take credit for this thought, but I will say that I do hold this opinion (e.g "Do not test private methods").

I have written several large-scale enterprise apps and have not ever needed to test a private method.  I'm saying this to brag or anything, I'm saying it to prove a point that it IS possible and SHOULD be a virtue worth striving for.

Having said all this, I am not prepared to say that testing private methods is ALWAYS wrong, but it's "ALMOST ALWAYS" wrong. There may be a few circumstances where it is beneficial or worth the risk of brittle tests to do this.

This is why I called it a code "stench".  It's worse than a smell, but it's not universally BAD.

RE: Internal methods

I feel mostly the same about this one, but it's not quite as dangerous as private methods, so I would tone down a the warning a little.  Generally, I manage to find a way to not have any internal methods also.

Internal methods usually denote direct coupling between concrete classes versus coupling through abstractions (public interfaces).  This creates a more brittle framework and prevents client code from overriding functionality if it needs to.

By using the Dependency Inversion Principle and Interface Segregation Principle, along with an IoC container, I seem to never need the 'internal' keyword and very rarely need the 'private' keyword, let alone tests for the same.

@mbratton

Protected methods are protected methods for a reason (i.e. you're exposing them to client code -- whether your own client code or the wide, wide world's client code).

So, I would give the following advice:

If it is 100% your code on both sides (framework/infrastructure code and client code) and the code in this project or other projects under your control (i.e. not code that will be used by a wide audience), I would concentrate on testing the classes that derive from the class-with-protected-members and you will essentially be testing the protected method(s) in question.

Having said that, if you find your tests repeating themselves for each sub-class, perhaps it would be better to have a test subclass that exposes the protected functionality and test that instead (to avoid duplication/repetition).

If this is a public framework where anyone and everyone is going to derive from your class and use this protected method, I would use the test subclass or a PartialMock (most mocking frameworks can do this -- see Rhino Mocks, Moq, TypeMock, etc).

@guy:

(1) "not testing a class"  -- I don't follow. Please elaborate.  In C#, it is not possible to write code without a class (for the most part, there are tricks), so I don't know how you could not test a class.  And I do this and it is TDD, so I don't know what you mean.

(2) "testing derived classes only" - Meaning: Test the public API first unless it gets repetitive and you start violating DRY, then consider testing the protected method directly

@Doug: Let me turn the question around on you:  Why must that task necessarily be private?

Frankly, I don't frequently find the need to make hardly anything private. Classes are small, singularly responsible, methods are small, and generally only a few methods per class. If I do need a private method, it's usually a small utility method of some sorts.

So your private methods are either a.) Used heavily by the public members in the class or b.) not used.

If a.) then they'll get tested when you test the public methods and b.) why are they there in the first place, let alone being tested?

And if your private methods are complex enough you really feel the need to test them separately, then you probably have a design smell or problem somewhere else you need to investigate.

This is the kind of stuff I like using TDD for, because it helps remind me when I'm making mistakes. When I have lots of stuff in a private method, TDD gently, but firmly, reminds me where I went wrong and how to correct it.