Unit Testing Principles

Writing good unit tests is challenging. Over time I’ve accumulated a number of principles about what to do in unit tests, as well as what not to do. This list is incomplete, but I’ll start it now.

• Test every possible code path. This means that for each ‘if’ statement, you’ll have at minimum 2 tests. Also known as code coverage.
• Write your tests first. TDD is a hard mindset to get into, but well worth it.
• If you have doubts about whether your code will do what you intend, you haven’t written enough tests.
• Your tests should never make calls to external resources such as a database, filesystem, etc.
• Every test should be self-sufficient and isolated. Your tests should not rely on other tests, or external factors. Be skeptical of setup/teardown code for a test harness class. (xUnit embraces this)
• Don’t test outside your code. There is no need to write test cases for an assembly you are dependent on.

This list will grow over time, especially if you have suggestions I might add.