[ThePadawan]

> What's the advantage of invariants over unit testing?

I studied Eiffel in university under Bertrand Meyer, and here's his (probably unique) point of view.

If someone hands you their library code and their unit tests, you have to understand their unit tests - which involves understanding why they chose the values that they did. There's also nothing stopping someone from testing the internals of their library as opposed to the publicly exposed behavior.

With contracts, imagine that to understand their library, you only see the method declarations with accompanying contracts. You don't see how HashSet.Add(x) is implemented, but you do see "if this.Contains(x) old(this.Count) == this.Count".

You don't have to see a test where this is tested with 5 example values, on an empty set, on a large set, whatever. You can rid yourself of that cognitive load by thinking, as sibling points out really well, declaratively over imperatively.