MSTest unit test passes when in “debug” mode, but fails final assertion in “run” mode

Question

Hi I have a unit test that goes attaches to an event and updates a counter based on a property of the eventArgs in the handler like so:

protected void UpdateCounts(object sender, EventArgs eventArgs)
{
  lock (lockobject)
  {
    Counts[eventArgs.Target]++;
  }
}

Counts is a static Dictionary resource that all unit tests in that class share. I assert that at the end of the test I assert the count to be 6 for a certain value of Target (Target is an enum). When I debug through this test, it always passes that final assertion, however, when I just run it without any breakpoints, the count for that value of Target can be 7 or 8, but never 6.

I realize that many threads trying to access an entry in a Dictionary may present a race condition, which is why I placed the lock around the increment. I also have a TestInitialize method which runs before every test gets run that looks like this:

[TestInitialize]
public void InitTest()
{
  foreach (TargetType x in Enum.GetValues(typeof(Target)))
  {
    Counts[x] = 0;
  }
}

Does anybody have any insight as to what is going on here?

Answer 1

Almost every time I have run into this issue it boils down to a race condition which is less likely to occur when you are debugging since you are slowing the execution down as you step through the code.

I would suggest adding debug or trace statements into the threads as they reach various stages of execution in order to identify where the race condition exists.