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[Charlie Poole <charlie@xxxxxxxxx>]

Although the 2.6 beta 2 has a fix for use of polling with After, this
particular issue hasn't come up before.

Although what you propose makes sense, it's not entirely clear how we
can implement it because it's entirely possible to be testing for a null
value...

   Assert.That(() => statusString, Is.Null.After(3000,100));

A silly example, but one we have to deal with.

One approach would be for DelayedConstraint to analyze all following
constraints to determine if a null reference will cause a problem. This
could be messy and violates the independence of composed constraints.
Another might be to catch the null reference exception and ignore it
during polling.

I think your first workaround is better, since it doesn't depend on
knowledge of NUnit's internals. The After constraint is in fact run on a
separate thread, but the main thread blocks until it is complete.

Charlie

-- 
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Developers, which is subscribed to NUnit V2.
https://bugs.launchpad.net/bugs/893919

Title:
  DelayedConstraint fails polling properties on references which are
  initially null

Status in NUnit V2 Test Framework:
  Triaged

Bug description:
  Having seen the great progress made on bug #890129, I've been using the After() constraint even more!
  (Still using v2.5.10.11092)

  I noticed a quirk with how (properties of) objects which are initially
  null are polled.

  An example:
      string statusString = null; // object starts off as null

      var worker = new BackgroundWorker();
      worker.RunWorkerCompleted += delegate { statusString = "finished"; /* object non-null after work */ };
      worker.DoWork += delegate { Thread.Sleep(TimeSpan.FromSeconds(1)); /* simulate work */ };
      worker.RunWorkerAsync();
           
      Assert.That(() => statusString, Has.Length.GreaterThan(0).After(3000, 100));

  The assert will fail after its first poll, reporting:
           System.ArgumentNullException : Value cannot be null.
           Parameter name: actual

  I assume this is because the 'statusString' object is null, so fetching the 'Length' property causes a ArgumentNullException.
  However, I'd expect that the test would at least continue to try to poll even if the value is initially null, in case it gets initialised before the timeout.

  A few simple workarounds exist:

  Adding a prior null check to the constraint:
      Assert.That(() => statusString, Is.Not.Null.With.Length.GreaterThan(0).After(3000, 100));

  Or adding another separate DelayedConstraint before the Assert which checks for non-null, like this:
      Assert.That(() => statusString, Is.Not.Null.After(3000, 100));
      Assert.That(() => statusString, Has.Length.GreaterThan(0).After(3000, 100));

  Perhaps that is because each constraint must pass or timeout before
  the next is run. The documentation doesn't specify whether each
  'After' assert is run on a separate thread or whether earlier ones
  block.

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