Well, I just tried out the C# version of this Kata with your using directives and Parse method body as-is (i.e. without any modifications whatsoever) and it passed all Submit tests as expected so I don't see what's the problem here. Keep in mind that the Node class displayed to you in the Kata Description is already defined in Preloaded so you don't need to redefine it yourself (in fact, you shouldn't).