So how do you write negative test cases with Pex? Here's a nice solution I was working on. I'm wondering

Traditional ExpectedExceptions style

Let's test the constructor a type Foo that takes a reference argument. We expect to throw a ArgumentNullException when null is passed. Therefore, we could write

[ExpectedException(typeof(ArgumentNullException))]
void Test() {
     new Foo(null);
}

or using xUnit style assertions,

void Test() {
     Assert.Throws(delegate { new Foo(null); });
}

Pex ExpectedExcetions style

Let's refactor our first test and push 'null' as a test parameter. We add an assertion after the constructor to make sure a null parameter never succeeds.

void Test(object input) {
    new Foo(input);
    Assert.IsNotNul(input); // we should never get here
}

The interesting part is that this test will not only test for the null value but also for the passing values as well. Moreover, there might be more checks over the input which might trigger other exceptions. In that case, additional asserts could be added -- or even better, centralized in an invariant method.

So, what do you think?