Question

Consider the following code:

private static BackgroundWorker bg = new BackgroundWorker();

static void Main(string[] args) {
  bg.DoWork += bg_DoWork;
  bg.ProgressChanged += bg_ProgressChanged;
  bg.WorkerReportsProgress = true;
  bg.RunWorkerAsync();

  Thread.Sleep(10000);
}

static void bg_ProgressChanged(object sender, ProgressChangedEventArgs e) {
  Console.WriteLine(e.ProgressPercentage);
  Thread.Sleep(100);
  Console.WriteLine(e.ProgressPercentage);
}

static void bg_DoWork(object sender, DoWorkEventArgs e) {
  for (int i = 0; i < 10; i++) {
    bg.ReportProgress(i);
  }
}

When run I get the following output:

0
1
1
2
0
3
2
3
5
4
4
6
5
7
7
8
6
9
8
9

I understand that the issue is a race condition between the threads that BackgroundWorker starts for each call to ReportProgress.

How can I make sure that the whole body of each bg_ProgressChanged gets executed in the order I have called them?
That is I would like to get

0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9

as a result.

Answer 1

BackgroundWorker raises ProgressChanged events on the current SynchronizationContext of the thread that called RunWorkerAsync().

The default SynchronizationContext runs callbacks on the ThreadPool without any synchronization.

If you use BackgroundWorker in a UI application (WPF or WinForms), it will use that UI platform’s SynchronizationContext, which will execute callbacks in order.