Task continuation on UI thread

Question

Is there a 'standard' way to specify that a task continuation should run on the thread from which the initial task was created?

Currently I have the code below - it is working but keeping track of the dispatcher and creating a second Action seems like unnecessary overhead.

dispatcher = Dispatcher.CurrentDispatcher;
Task task = Task.Factory.StartNew(() =>
{
    DoLongRunningWork();
});

Task UITask= task.ContinueWith(() =>
{
    dispatcher.Invoke(new Action(() =>
    {
        this.TextBlock1.Text = "Complete"; 
    }
});

Answer 1

Call the continuation with TaskScheduler.FromCurrentSynchronizationContext() :

    Task UITask= task.ContinueWith(() =>
    {
     this.TextBlock1.Text = "Complete"; 
    }, TaskScheduler.FromCurrentSynchronizationContext());

This is suitable only if the current execution context is on the UI thread.

Answer 2

With async you just do:

await Task.Run(() => do some stuff);
// continue doing stuff on the same context as before.
// while it is the default it is nice to be explicit about it with:
await Task.Run(() => do some stuff).ConfigureAwait(true);

However:

await Task.Run(() => do some stuff).ConfigureAwait(false);
// continue doing stuff on the same thread as the task finished on.

Answer 3

If you have a return value you need to send to the UI you can use the generic version like this:

This is being called from an MVVM ViewModel in my case.

var updateManifest = Task<ShippingManifest>.Run(() =>
    {
        Thread.Sleep(5000);  // prove it's really working!

        // GenerateManifest calls service and returns 'ShippingManifest' object 
        return GenerateManifest();  
    })

    .ContinueWith(manifest =>
    {
        // MVVM property
        this.ShippingManifest = manifest.Result;

        // or if you are not using MVVM...
        // txtShippingManifest.Text = manifest.Result.ToString();    

        System.Diagnostics.Debug.WriteLine("UI manifest updated - " + DateTime.Now);

    }, TaskScheduler.FromCurrentSynchronizationContext());

Answer 4

I just wanted to add this version because this is such a useful thread and I think this is a very simple implementation. I have used this multiple times in various types if multithreaded application:

 Task.Factory.StartNew(() =>
      {
        DoLongRunningWork();
        Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal, new Action(() =>
              { txt.Text = "Complete"; }));
      });

Answer 5

Got here through google because i was looking for a good way to do things on the ui thread after being inside a Task.Run call - Using the following code you can use await to get back to the UI Thread again.

I hope this helps someone.

public static class UI
{
    public static DispatcherAwaiter Thread => new DispatcherAwaiter();
}

public struct DispatcherAwaiter : INotifyCompletion
{
    public bool IsCompleted => Application.Current.Dispatcher.CheckAccess();

    public void OnCompleted(Action continuation) => Application.Current.Dispatcher.Invoke(continuation);

    public void GetResult() { }

    public DispatcherAwaiter GetAwaiter()
    {
        return this;
    }
}

Usage:

... code which is executed on the background thread...
await UI.Thread;
... code which will be run in the application dispatcher (ui thread) ...