Never - I always attempt to make an end to end implementation. Once the code works I go back an examine the what I did and try to make it simpler. This often re

Never - I always attempt to make an end to end implementation. Once the code works I go back an examine the what I did and try to make it simpler. This often requires refactoring and changing API's etc. Writing a bunch of test would simply add inertia to that process. After I am satisfied with the code or get bug reports I will add test. Test code is code and often has bugs. Every line of code I write is a liability so I try to limit it to necessary things. I have never seen the write test approach work. Once all the test code is written people become reluctant to change things because they have to change both the code and test code doubling the work/time. One just ends up with well tested but not so good code. If you a M$ and can afford to pair program it might be more feasible.