NUnit vs. MbUnit vs. MSTest vs. xUnit.net

NUnit is probably the most supported by the 3rd party tools. It's also been around longer than the other three.

I personally don't care much about unit test frameworks, mocking libraries are IMHO much more important (and lock you in much more). Just pick one and stick with it.

I wouldn't go with MSTest. Although it's probably the most future proof of the frameworks with Microsoft behind it's not the most flexible solution. It won't run stand alone without some hacks. So running it on a build server other than TFS without installing Visual Studio is hard. The visual studio test-runner is actually slower than Testdriven.Net + any of the other frameworks. And because the releases of this framework are tied to releases of Visual Studio there are less updates and if you have to work with an older VS you're tied to an older MSTest.

I don't think it matters a lot which of the other frameworks you use. It's really easy to switch from one to another.

I personally use XUnit.Net or NUnit depending on the preference of my coworkers. NUnit is the most standard. XUnit.Net is the leanest framework.

I know this is an old thread, but I thought I'd post a vote for xUnit.NET. While most of the other testing frameworks mentioned are all pretty much the same, xUnit.NET has taken a pretty unique, modern, and flexible approach to unit testing. It changes terminology, so you no longer define TestFixtures and Tests...you specify Facts and Theories about your code, which integrates better with the concept of what a test is from a TDD/BDD perspective.

xUnit.NET is also EXTREMELY extensible. Its FactAttribute and TraitAttribute attribute classes are not sealed, and provide overridable base methods that give you a lot of control over how the methods those attributes decorate should be executed. While xUnit.NET in its default form allows you to write test classes that are similar to NUnit test fixtures with their test methods, you are not confined to this form of unit testing at all. You are free to extend the framework to support BDD-style Concern/Context/Observation specifications, as depicted here.

xUnit.NET also supports fit-style testing directly out of the box with its Theory attribute and corresponding data attributes. Fit input data may be loaded from excel, database, or even a custom data source such as a Word document (by extending the base data attribute.) This allows you to capitalize on a single testing platform for both unit tests and integration tests, which can be huge in reducing product dependencies and required training.

Other approaches to testing may also be implemented with xUnit.NET...the possibilities are pretty limitless. Combined with another very forward looking mocking framework, Moq, the two create a very flexible, extensible, and powerful platform for implementing automated testing.