Unit Testing with Pex and Code Contracts

Question

I have the following method:

public static DateTime SubQtrs( this DateTime dt, int qtrs )
{
    Contract.Requires( qtrs > -1 );
    Contract.Requires( ( qtrs * 3 ) >= -120000 && ( qtrs * 3 ) <= 120000 );
              // do something
     }

I created a paramertized unit test and used Pex to come up with some unit test. As expected, Pex tested the contract constraints and passed in a qtrs value that violated the contract. I told the unit test to expect the exception with: [ExpectedException(typeof(TraceAssertionException))] and everything passed.

Now when I just run the unit test with the MS Test Harness (without Pex), I get an error: Descrtipion: Precondition failed: (qtrs * 3 >= -120000 && ......

It gives me the stack trace basically telling me the contracthelper failed. Then I'm given the choice to Abort, Retry, or Ignore.

Either way, the test fails and I get: Failed SubQtrsThrowsTraceAssertionException818 DGALib.Extensions.Tests Test method System.ExtensionMethodsTest.SubQtrsThrowsTraceAssertionException818 threw exception System.ArgumentOutOfRangeException, but exception Microsoft.Pex.Framework.Exceptions.TraceAssertionException was expected. Exception message: System.ArgumentOutOfRangeException: Months value must be between +/-120000. ...

Why is the unit test not recognizing the code contract error anymore?

Answer 1

It seems the tests that were throwing up that screen were missing [HostType("Moles")].

I compared a test that was causing the problem with another test that was also supposed to throw a trace assertion and wasn't causing a problem. The only difference was the one causing the problem was missing [HostType("Moles")]. As soon as I added that attribute, the test worked.

There error must about the contracts must have been bubbling up from somewhere else and sending me on a wild goose chase.

Pex must have created that test before I started to add moles to the rest of the testing scenarios.