NUnit is a unit test harness for use with .NET languages.  A test harness allows an application to start at an alternate entry point (rather than a main() method); the harness then controls the flow of execution through a series of test method written by you.  You write a test class that contains any required initialization for your application, as well as test methods, and then run the test harness against your compiled executable file.  The test harness uses reflection to reach inside the file and test your methods.

Here's how to download and install NUnit on your PC for VS.NET 2005 and the .NET Framework 2.0.

Coding with a good test harness means you can always perform automated regression (unit) testing on your code with the click of a mouse; this helps detect whether future code modifications have broken existing code.  Recent "best practice" is to code in this order: 1) write an empty method stub, 2) write a test for the method (this test will fail at this point), and 3) add the real logic to the new method so the test passes.

We are also supposed to automate testing of exceptions.  If I know that one of my methods can throw a certain kind of exception, I am supposed to write a test that deliberately causes the exception to occur.  Knowing that exception handling is working as expected is as important as knowing how methods perform under normal circumstances.

Here's a Counter sample project (here's the counter class code and its test class) that illustrates the use of NUnit.  Note the three different test methods which test to make sure that an exception occurred.  When NUnit runs this code, the output looks like this:

The list below (obtained from the test harness console output) shows the order in which the methods in the test class will be called when NUnit controls how the application runs:

TestFixtureSetUp

SetUp
Test001IsNull
TearDown

SetUp
Test002IsNotNull
TearDown

SetUp
Test003IncrementOrigValue
TearDown

SetUp
Test004DecrementOrigValue
TearDown

SetUp
Test005AreSame
TearDown

SetUp
Test006AreEqual
TearDown

SetUp
Test007ExceptionThrown
TearDown

SetUp
Test008ExceptionThrown
TearDown

SetUp
Test009ExceptionThrown: System.Exception: messing up now
   at counter.Counter.ThrowAnException()
   at counter.TestFixtureSetUpAndTearDownTest.Test009ExceptionThrown()
TearDown

SetUp
Test010ExceptionThrown
TearDown

TestFixtureTearDown