Important testing principles
One essential thing to remember when writing tests is to test use cases, not the code itself; we are testing features’ correctness, not code correctness. Of course, if the expected outcome of a feature is correct, that also means the codebase is correct. However, it is not always true for the other way around; correct code may yield an incorrect outcome. Also, remember that code costs money to write while features deliver value.
To help with that, the test requirements usually revolve around the inputs and outputs. When specific values go into your subject under test, you expect particular values to come out. Whether you are testing a simple Add method where the ins are two or more numbers and the out is the sum of those numbers, or a more complex feature where the ins come from a form and the out is the record getting persisted in a database, most of the time, we are testing the ins and outs.
That’s the first principle you must know...