c# - how can i force await to continue on the same thread?

Question

await does not guarantee continuation on the same task for spawned tasks:

private void TestButton_Click(object sender, RoutedEventArgs e)
{
    Task.Run(async () =>
    {
        Debug.WriteLine("running on task " + Task.CurrentId);
        await Task.Delay(TimeSpan.FromMilliseconds(100));
        Debug.WriteLine("running on task " + Task.CurrentId);
    });
}

The output of this is:

running on task 1
running on task

so we can see that not only the execution has moved to another task, but also to the UI-thread. How can i create a dedicated task, and enforce await to always continue on this task? Long-running tasks don't do this either.

I have seen several SynchronizationContext implementations, but so far none of them worked, in this case because it uses threads and System.Threading.Thread is not available for uwp.

Answer 1

so we can see that not only the execution has moved to another task, but also to the UI-thread.

No, it's not on the UI thread. It's just technically not on a task, either. I explain why this happens in my blog post on Task.CurrentId in async methods.

How can i create a dedicated task, and enforce await to always continue on this task? Long-running tasks don't do this either.

You're on the right track: you need a custom SynchronizationContext (or a custom TaskScheduler).

I have seen several SynchronizationContext implementations, but so far none of them worked, in this case because it uses threads and System.Threading.Thread is not available for uwp.

Try out mine. It should work on UWP 10.0.