BackgroundWorker not working with TeamCity NUnit runner

Question

I'm using NUnit to test View Models in a WPF 3.5 application and I'm using the BackgroundWorker class to execute asynchronous commands.The unit test are running fine with the NUnit runner or ReSharper runner but fail on TeamCity 5.1 server.

How is it implemented :

I'm using a ViewModel property named IsBusy and set it to false on BackgroundWorker.RunWorkerCompleted event. In my test I'm using this method to wait for the BackgroundWorker to finish :

protected void WaitForBackgroundOperation(ViewModel viewModel)
{
    Console.WriteLine("WaitForBackgroundOperation 1");
    int count = 0;
    while (viewModel.IsBusy)
    {
        Console.WriteLine("WaitForBackgroundOperation 2");
        RunBackgroundWorker();
        Console.WriteLine("WaitForBackgroundOperation 3");

        if (count++ >= 100)
        {
            Assert.Fail("Background operation too long");
        }
        Thread.Sleep(1000);

        Console.WriteLine("WaitForBackgroundOperation 4");
    }
}

private static void RunBackgroundWorker()
{
    Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.Background, new ThreadStart(delegate { }));
    System.Windows.Forms.Application.DoEvents();
}

Well, sometimes it works and sometimes it hangs the build. I suppose it's the Application.DoEvents() but I don't know why...

Edit: I added some traces (see code above) and in the log I have :

WaitForBackgroundOperation 1 
WaitForBackgroundOperation 2 
WaitForBackgroundOperation 2 
WaitForBackgroundOperation 2
...

How is it possible ?!

Answer 1

You are launching a bunch of background tasks, one per second. Move the RunBackgroundWorker above the loop.

protected void WaitForBackgroundOperation(ViewModel viewModel)
{
    Console.WriteLine("WaitForBackgroundOperation 1");
    RunBackgroundWorker();
    Thread.Sleep(100); // wait for thread to set isBusy, may not be needed
    int count = 0;
    while (viewModel.IsBusy)
    {
        Console.WriteLine("WaitForBackgroundOperation 2");
        if (count++ >= 100)
        {
            Assert.Fail("Background operation too long");
        }
        Thread.Sleep(1000);
        Console.WriteLine("WaitForBackgroundOperation 3");
    }
}

private static void RunBackgroundWorker()
{
    Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.Background, new ThreadStart(delegate { }));
}

The DoEvents should not be needed.