Should I unit-test with data that should not be passed in a function (invalid input)?

Question

I am trying to use TDD for my coding practice. I would like to ask should I test with a data that should not happen in a function BUT this data may possibly break your progr

Answer 1

If the expected outcome is that an exception is thrown with invalid input values, then a test that the exceptions get properly thrown would be appropriate.

Edit:

As I noted in my comment below, if these cases will break your application, you should throw an exception. If it really is logically impossible for these cases to occur, then I would say no, you don't need to throw an exception, and you don't need test cases to cover it.

Note that if your system is well componentized, and this function is one component, the fact that it is logically impossible now doesn't mean it will always be logically impossible. It may be used differently down the road.