Structural equality for better tests by Mark Seemann A Fluent Builder as a Value Object? If you've read a bit about unit testing, test-driven development, or other kinds of developer testing, you've probably come across a phrase like this: Test behaviour, not implementation. It's often taken to mean something like behaviour-driven development (BDD), and that's certainly one interpretation. I've no problem with that. My own Pluralsight course Outside-In Test-Driven Development shows a similar technique. It'd be a logical fallacy, however, to thereby conclude that you can only apply that ideal in the large, but not in the small. That it's only possible to do it with coarse-grained tests at the boundary of the system, but not with unit testing.