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

In short, if it can break, then you should test it. Also validate data at the earliest point possible.

The answer depends on whether you control the inputs passed to Robot. If Robot is an internal class (C#) ; values only flow in from RobotClientX which is a public type. Then I'd put the guard checks in RobotClientX, write tests for it. I'd not write tests for Robot, because invalid values cannot materialize in-between. e.g. if I put my validations in the GUI such that all invalid values are filtered off at the source, then I don't check for invalid values in all classes below the GUI (Unless I've also exposed a public API which bypasses the GUI).

On the other hand, if Robot is publicly visible i.e. Anyone can call Robot with any value that they please, then I need tests that document it's behavior given specific kinds of input.. invalid being one of them. e.g. if you pass an out-of-range value, it'd throw an ArgumentException.