C# Unit Testing warn instead of fail

It sounds like you are attempting to test too many things in a single test.

If a precondition isn't met, then presumably the rest of the test will not pass either. I'd prefer to end the test as soon as I know things aren't what I expect.

The concepts of unit testing are Red fail, Green pass. I know MSTest also allows for a yellow, but it isn't going to do what you want it to. You can do an Assert.Inconclusive to get a yellow light. I have used this when I worked on a code base that had a lot of integration tests that relied on specific database data. Rather than have the test fail, I started having the results be inconclusive. The code might have worked just fine, but the data was missing. And there was no reason to believe the data would always be there (they were not good tests IMO).

If you are using Gallio/MbUnit, you can use Assert.Multiple to achieve what you want. It captures the failing assertions but does not stop the execution of the test immediately. All the failing assertions are collected and reported later at the end of the test.

[Test]
public void MultipleAssertSample()
{
   Assert.Multiple(() =>
   {
      Assert.Fail("Boum!");
      Assert.Fail("Paf!");
      Assert.Fail("Crash!");
   });
}

The test in the example above is obviously failing but what's insteresting is that the 3 failures are shown in the test report. The execution does not stop at the first failure.