Test Driven Development (a.k.a. Test-first design) is one of the core programming practices of XP (Extreme Programming). Many of us have learned over the years the value of writing automated tests for our code. We have also learned the difficulty of writing tests after code is already in place. Test-first design takes a different, extreme approach to ensure that we test all code all the time.

We write tests not as an afterthought to ensure our code works, but instead as just part of the everyday, every minute way of building software. Instead of writing our detailed design specifications on paper, we write them in code. Instead of first striving to perfectly design a system on paper, we use tests to guide our design. Instead of coding for hours at a stretch, only to find our planning going awry, we use test-first design to pace ourselves, always assuring that we are moving forward correctly with each passing minute.

We use NUnit framework to do test-driven development.

The steps:

The rules:

The benefits:

Code is written so that modules are testable in isolation. Code written without tests in mind is often highly coupled, a big hint that you have a poor object-oriented design. If you have to write tests first, you'll devise ways of minimizing dependencies in your system in order to write your tests.

The tests act as system-level documentation. They are the first client of your classes; they show how the developer intended for the class to be used.

The system has automated tests by definition. As your system grows, running full regression tests manually will ultimately take outrageous amounts of time.