tdd - testing an internal class

Question

how to write unit tests to internal classes ???

Answer 1

You write tests which specify the behaviour of the top-level class' external interface. Whether that class uses internal classes to implement that behaviour or not, is an implementation detail of the class, and the tests don't need to know anything about it.

If the internal class cannot be adequately tested through the top-level class' interface, then it's usually best to move the internal class out and test it directly as a new top-level class. Wanting to test internal classes is a code smell that the internal class might be significant enough to be a top-level class.