Although developers have been unit testing their code for years, it was typically performed after the code was designed and written. As a great number of developers can attest, writing tests after the fact is difficult to do and often gets omitted when time runs out. Test-driven development (TDD) attempts to resolve this problem and produce higher quality, well-tested code by putting the cart before the horse and writing the tests before we write the code. One of the core practices of Extreme Programming (XP), TDD is acquiring a strong following in the Java community, but very little has been written about doing it in .NET.
What Are Unit Tests?
According to Ron Jeffries, Unit Tests are "programs written to run in batches and test classes. Each typically sends a class a fixed message and verifies it returns the predicted answer." In practical terms this means that you write programs that test the public interfaces of all of the classes in your application. This is not requirements testing or acceptance testing. Rather it is testing to ensure the methods you write are doing what you expect them to do. This can be very challenging to do well. First of all, you have to decide what tools you will use to build your tests. In the past we had large testing engines with complicated scripting languages that were great for dedicated QA teams, but weren't very good for unit testing. What journeyman programmers need is a toolkit that lets them develop tests using the same language and IDE that they are using to develop the application. Most modern Unit Testing frameworks are derived from the framework created by Kent Beck for the first XP project, the Chrysler C3 Project. It was written in Smalltalk and still exists today, although it has gone through many revisions. Later, Kent and Erich Gamma (of Patterns fame) ported it to Java and called it jUnit. Since then, it has been ported to many different languages, including C++, VB, Python, Perl and more.