Using both MSTest and NUnit?

You can certainly have one test project using MSTest and another using NUnit. If you want to mix both in the same project, you'll have to stick to common features and use compiler directives and using statements to "rename" and compile to certain ones.

You'd have to run both nunit and mstest though to figure out if all your tests really pass, so I'd strongly advise against trying to use both. Pick one.

If you want to run tests on a build server, probably the best bet is NUnit, unless you want to install VS on your build server as well. TFS may work without installing VS though, but you'll have to check your documentation.

I personally don't like the idea of mixing two frameworks like it's mentioned in the article you are referring to.

A possible condition to make your unit test run under both test frameworks could be that you do not want or can not install Visual Studio on your continuous integration server.

Just for clarification, MSTest is not the Visual Studio Unit Testing Framework defined in the Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll assembly.

To be able to let your unit test run under both frameworks you would need to define a build constant (here NUNIT) and with the help of preprocessor directives and namespace aliases you would end up with a snippet like this:

#if !NUNIT
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Category = Microsoft.VisualStudio.TestTools.UnitTesting.DescriptionAttribute;
#else
using NUnit.Framework;
using TestInitialize = NUnit.Framework.SetUpAttribute;
using TestContext = System.Object;
using TestProperty = NUnit.Framework.PropertyAttribute;
using TestClass = NUnit.Framework.TestFixtureAttribute;
using TestMethod = NUnit.Framework.TestAttribute;
using TestCleanup = NUnit.Framework.TearDownAttribute;
#endif

I believe I came across this idea first through this MSDN Magazine article. Another worthwhile blog post covering this topic would be:

• Using nUnit AND Mstest (Visual Studio Unit Testing) together in perfect harmony

To your last question: The Fluent Assertions project seems to be a good library. I do not see any real reasons why you shouldn't use it if you like the fluent style of it.

Add a reference to nunit.framework and the following line to the top of your MSTest tester class...

using NAssert = NUnit.Framework.Assert;

Now you can use either...

// Test whether a new SimplexInterpreter was created
[TestMethod]
public void SimplexInterpreterConstructorTest()
{
    Assert.IsNotNull(target);
    NAssert.IsNotNull(target);
}