XUnit – MemberDataAttribute and generic type inheritance

1. Introduction.

When I was writing Cake.Intellisense I was playing a lot with Roslyn syntax rewriters. The one thing which was pretty important for me at this time was a reliable set of tests which can quickly show if rewritten code is valid C# code and if it matches my expectations. As I don’t like to repeat my code I wanted to have a base test class which can be provided with different test cases from inherited classes. The initial implementation looked as follows

As you can see I have a generic base test class which holds test cases in a static field TestCases. The field is static as it is required by XUnit, this solution is perfectly valid because a static field is shared across all instances of the same type. Having a base test class ready I wanted to write a set of unit tests for following syntax rewriter

The rewriter shown above is a very simple piece of code which replaces internal class modifier with a public one. Writing the tests for that class was supposed to be very easy, I just had to provide test cases with input code and expected result.

Unfortunately running the tests resulted in InvalidOperationException being thrown from XUnit

If you look closely at the message you will notice that the TestCases field used for providing data was not initialized, which means that the static constructor of class InternalToPublicClassModifierSyntaxRewriterTests was not run. That was pretty surprising for me that is why I wanted to investigated the issue a bit more.

2. Investigation.

I started from going back to the basics. As we probably all know, the static constructor is triggered when one of the following events occur:

  • An instance of the class type is created.
  • Any of the static members of the class type are referenced

The first condition is not met because instance of the test class is created after test cases are enumerated. That is why member data fields have to be a static fields/properties etc. When it comes to second condition the situation in here is a bit more complicated but if you take a look at the MemberDataAttributeBase class

you will notice that member data looks for properties in DeclaringType of method. In my case declaring type is


So when XUnit accesses TestCase property the static constructor from InternalToPublicClassModifierSyntaxRewriterTests class will not be called either as the types does not match.

3. Solution.

In order to bend the behavior of XUnit to my needs, I had to ensure that the static constructor from inherited class is called. Fortunately, workaround is pretty straightforward – I had to introduce a custom MemberDataAttribute which calls static constructor from ReflectedType manually

Now all I have to do is use newly created attribute in CSharpSyntaxRewriterTest test class

From now on all of the test cases will be run correctly

Source code for this post can be found here

XUnit – MemberDataAttribute and generic type inheritance