How many times we have to develop some sort of applications like a Console or Forms applications to test a class library.
Unit testing is a standard way of testing; .net code can be tested using nUnit
nUnit is a tool that runs the tests you create and show the result is avery organized way.
TestDriven.NET is a tool that enable runnig the tests from within Visual Studio
Read the tutorial on nUnit web site ;it is a fast start