I agree that contracts don't offer something stronger than unit test. They offer something different. Of course you can't write the unit test that I asked and t

I agree that contracts don't offer something stronger than unit test. They offer something different. Of course you can't write the unit test that I asked and that was the point. But it would be kind of pointless too to write the test that checks that an exception is thrown when sqrt is called with a negative argument. It's trivial to see that happens by looking at the code. How sqrt fails is not interesting either, as you are not supposed to recover from that. A pre-condition is a way to specify that. It says: "Don't call me with a negative argument, just don't."