NUnit asserts in separate thread

The NUnit test runner executable is configured to ignore exceptions on non-test threads. You can configure this behaviour via app.config configuration element:

legacyUnhandledExceptionPolicy

You can configure non-test threads to cause unhandled exceptions, and so tests will then fail. The caveat is that non-test thread exceptions may be raised whilst another test is running so you may not get an accurate picture of which test failed. Also, as this will be an unhandled exception, test execution will stop at the point the exception happens, rather then continue which is the normal behaviour.

Better than ignoring though in my opinion.

The following article goes into some detail on the topic for the ReSharper test runner, but the principles are the same.

ReSharper test runner – hidden thread exceptions

If you do have multi-threading aspects, from an assertion point of view, I find it better to set flags etc on the non-test thread, wait for the non-test thread to finish, and then assert the state on the test thread. In this way assertion exceptions are raised on the main thread and tests run as expected.

Contrived Example

    [Test]    public void Test()    {        Exception ex = null;        Action test = () =>        {            Console.WriteLine("Async");            ex = new InvalidOperationException();        };        test.BeginInvoke(test.EndInvoke, null).AsyncWaitHandle.WaitOne();        Assert.That(ex, Is.Null, "Exception did not happen.");    }

Assert.IsTrue(false);

Will throw an exception before the DoesNotThrow call. But NUnit does not work over multiple threads. Cross-threads is more integration then UnitTest. In general I would suggest to avoid to expose the threads/tasks from outside and make an inner behaviour of the used object - so that you can mock this behaviour for testing purposes.