Extend xUnit.NET to use custom code when processing a class and locating test methods

Question

I'm a big fan of the xUnit.NET framework; I find it light, simple, clean, and extensible.

Now let's say that I have a class like so:

public class AdditionSpecification
{
  static int result;

  public void Because()
  {
    result = 2 + 2;
  }

  public void Result_is_non_zero()
  {
    Assert.True(result <> 0);
  }

  public void Result_is_correct()
  {
    Assert.Equal(4, result);
  }
}

With the test class above I want xUnit.NET to see 2 test cases and to run the Because() method before each of them.

Leaving aside any issues you may have with my class or method names, the structure of this test/specification, the xUnit.NET framework, or BDD, here's my question:

How can I tell xUnit.NET that I want to customize how it identifies and executes test methods out of this class without using a custom [Fact]-like attribute on each target test method?

I know that I can derive from BeforeAfterAttribute to decorate each test method with custom before and after execution. How can i do this at the class level? Do i have to write a custom runner?

Answer 1

So it turns out that I was looking for the ITestClassCommand.EnumerateTestMethods() method.

1. The default xUnit.NET test runner will iterate over all the classes in your test assembly.
2. For each one it will check for a RunWithAttribute; that's your chance to override the ITestClassCommand implementation that is used to identify methods containing tests. (RunWithNUnit is a good example)
3. ITestClassCommand.EnumerateTestMethods() is called to process the test class and return an IEnumerable of test methods.
4. each test IMethodInfo is then passed to ITestClassCommand.EnumerateTestCommands(IMethodInfo testMethod) to get the IEnumerable of ITestCommands
5. each ITestCommand is then executed and given the opportunity to return a result.

In the case of my example above, I would need something like:

[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
public class RunWithMyTestClassCommandAttribute : RunWithAttribute
{
   public RunWithMyTestClassCommandAttribute()
               : base(typeof(MyTestClassCommand)) {}
}

Then I could decorate my above example with:

[RunWithMyTestClassCommand]
public class AdditionSpecification
{
  static int result;

  public void Because()
  {
    result = 2 + 2;
  }

  public void Result_is_non_zero()
  {
    Assert.True(result <> 0);
  }

  public void Result_is_correct()
  {
    Assert.Equal(4, result);
  }
}

Finally, in MyTestClassCommand, I get to opportunity between EnumerateTestMethods() and EnumerateTestCommands(IMethodInfo testMethod) to use whatever logic I want to locate and construct ITestCommand instances that get executed as individual tests.

BTW, in the process of researching this issue, I ran into a small bug in the xUnit.NET framework where a custom IMethodInfo generated by EnumerateTestMethods() never showed up in EnumerateTestCommands(..) because it was being unwrapped and rewrapped by the test runner or one of it's factories.

I submitted this issue to the xUnit project on codeplex and it was corrected on May 30th, 2009 for xUnit.NET 1.5 CTP 2