Heaps of good stuff in this article from Patrick Smacchia about writing tests

Best bits

The tests should be written before the functionality that is being tested’.

I don’t agree with that. Tests for a portion of code should be written at the same time as the code itself.

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About Code contracts and assertions (Debug.Assert)

Assertions in tests are not much different than code contract assertions.

Although Code Contracts are way slower on compile time, which is #pain.

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It is a good practice to tag a class that has been 100% covered with an arbitrary [FullCovered] attribute, because it documents that the class is, and must remain 100% covered

Great idea. The build should fail if someone is not following that rule (marked by the attribute)

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…writing a complete test suite naturally leads to good design, enforcing low coupling and high cohesion.

Great recap!

Full article: On Writing Unit Tests for C#
http://www.simple-talk.com/dotnet/.net-framework/on-writing-unit-tests-for-c/