How often should we write unit tests?

Question

I am recently introduced to the test-driven approach to development by my mentor at work, and he encourages me to write an unit-test whenenver "it makes sense." I understand some benefits of having a throughout unit-test suite for both regression testing and refractoring, but I do wonder how often and how throughout we should write unit-test.

My mentor/development lead asks me to write a new unit test-case for a newly written control flow in a method that is already being tested by the exsisting test class, and I think it is an overkill. How often do you write your unit tests, and how detailed do you think your unit tests should be? Thanks!

Answer 1

Technically, if you're following strict Test Driven Development...you should be writing your Unit Tests after you have spec'd any portion of your application. You should be going from:

User Requirements -> Function Design -> Unit Tests -> Code

Remembers, Tests come before Code in TDD.