Often I am asked variations on this question: Should I unit test private methods?
The Visual Studio Team Test blog describes the Publicize testing technique in Visual Studio as one way to unit test private methods. There are other methods.
As a rule of thumb: Do not unit test private methods.
Encapsulation
The concept of encapsulation means that a class’s internal state and behavior should remain “unpublished”. Any instance of that class is only manipulated through the exposed properties and methods.
The class “publishes” properties and methods by using the C# keywords: public, protected, and internal.
The one keyword that says “keep out” is private. Only the class itself needs to know about this property or method. Since any unit test ensures that the code works as intended, the idea of some outside code testing a private method is unconventional. A private method is not intended to be externally visible, even to test code.
However, the question goes deeper than unconventional. Is it unwise to unit test private methods?
Yes. It is unwise to unit test private methods.
Brittle Unit Tests
When you refactor the code-under-test, and the private methods are significantly changed, then the test code testing private methods must be refactored. This inhibits the refactoring of the class-under-test.
It should be straightforward to refactor a class when no public properties or methods are impacted. Private properties and methods, because they are not intended to be directly called, should be allowed to freely and easily change. A lot of test code that directly calls private members causes headaches.
Avoid testing the internal semantics of a class. It is the published semantics that you want to test.
Zombie Code
Some dead code is only kept alive by the test methods that call it.
If only the public interface is tested, private methods are only called thorough public-method test coverage. Any private method or branch within the private method that cannot be reached through test coverage is dead code. Private method testing short-circuits this analysis.
Yes, these are my views on what might be a hot topic to some. There are other arguments, pro and con, many of which are covered in this article: http://www.codeproject.com/Articles/9715/How-to-Test-Private-and-Protected-methods-in-NET
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Tested, working code trumps rules about privacy every time.
Feathers says it best, in “Working Effectively with Legacy Code”
“This question comes up over and over again from people new to unit testing: “How do I test private methods?”. Many people spend a lot of time trying to figure out how to get around this problem, but […] the real answer is that if you have the urge to test a private method, the method shouldn’t be private; if making the method public bothers you, chances are, it is because it is part of a separate responsibility: it should be on another class.”