From #1279. TestCaseAttribute supports a number of 'special-cases' that TestCaseSourceAttribute does not - principally optional arguments and the params keyword.

This is managed within TestCaseAttribute.GetParametersForTestCase(IMethodInfo method). This functionality could probably be shared by both attributes, however, there's some additional complexity in how TestCaseSourceAttribute.GetTestCasesFor(IMethodInfo method) derives the type of it's parameters which will need modification.