unexpected async await behavior in mstest unit test - testing

Question

I'm having newbie problems testing an async method. I've read a few other posts, including How to correct write test with async methods?, and I think I'm doing things correctly, but I'm clearly missing something.
Here's the code I'm trying to test:
/// <summary>
/// Temporary method used to simulate the time required to generate a report.
/// </summary>
public virtual async Task GenerateTestReportAsync()
{
++ActiveTaskCounter;
await Task.Delay(2000);
--ActiveTaskCounter;
}
And here's the test (yes, I could/should improve it by storing the ActiveTaskCounter values in a list, but that's beside the point):
[TestMethod]
public async Task GenerateTestReportAsyncUpdatesActiveTaskCounter()
var bayViewModel = CreateBayViewModel();
var invocationCount = 0;
bayViewModel.PropertyChanged += (sender, args) =>
{
if (args.PropertyName == "ActiveTaskCounter")
{
++invocationCount;
}
};
await bayViewModel.GenerateTestReportAsync();
//Thread.Sleep(2100); // TODO: Why is this needed?
Assert.AreEqual(2, invocationCount);
Assert.AreEqual(0, bayViewModel.ActiveTaskCounter);
}
The test always passes with the Thread.Sleep uncommented. It always fails with the Thread.Sleep commented, as shown above. Why is this? I assumed the code below the "await bayViewModel.GenerateTestReportAsync()" wouldn't even execute until GenerateTestReportAsync was really finished.
BTW, I'm using:
Microsoft Visual Studio Ultimate 2012
Version 11.0.51106.01 Update 1
Microsoft .NET Framework
Version 4.5.50709
Thanks for any help you can provide! This has me stumped :-|
EDIT: Here's a minimal repro of the view model class...
using System.ComponentModel;
using System.Threading.Tasks;
public class BayViewModel : INotifyPropertyChanged
{
public BayViewModel()
{
PropertyChanged = (sender, args) => { };
}
public event PropertyChangedEventHandler PropertyChanged;
private int _activeTaskCounter;
public int ActiveTaskCounter
{
get
{
return _activeTaskCounter;
}
set
{
_activeTaskCounter = value;
PropertyChanged(this, new PropertyChangedEventArgs("ActiveTaskCounter"));
}
}
public virtual async Task GenerateTestReportAsync()
{
++ActiveTaskCounter;
await Task.Delay(2000);
--ActiveTaskCounter;
}
}