XUnit Traits and Categories

One of the things I love about xUnit.net is how small the feature set is while allowing you to extend it with the minimum of effort.

One project had a number of unit tests that needed to touch the file system - write a file and then read it back in. The functionality I wanted to test was interesting enough to warrant a set of tests and yet if I stubbed out the file system part I was removing most of the value from having the tests in the first place.

The thing to do in this case is to segregate the tests into fast, genuine unit tests and slow, nasty, evil integration tests. One approach is to build them into separate assemblies and run them as two separate tests.

The other approach is to put them into categories and run them according to the context. Perhaps, all tests are run on a developer's local machine and only the fast unit tests are run in an automated build.

In xUnit, this segregation is achieved using a Trait attribute. xUnit test runners can filter tests based on the name "Category".

I created a set of traits to be applied to fast and slow unit tests as appropriate.

    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
    public class InMemoryTestAttribute : TraitAttribute
    {
       public InMemoryTestAttribute()
           : base("Category", "Fast")
       {
       }
    }
    
    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
    public class TouchingFileSystemTestAttribute : TraitAttribute
    {
        public TouchingFileSystemTestAttribute()
            : base("Category", "Slow - FileSystem")
        {
        }
    }
    
    public class CompilerFacts
    {
        [Fact]
        [InMemoryTest]
        public void Compiler_Created_With_Default_Properties()
        {
            // ... fast test here
        }
        
        [Fact]
        [TouchingFileSystemTest]
        public void Compiler_Writes_File_In_Correct_Location()
        {
            // ... slow test here
        }
    
    }