How to create a task (TPL) running a STA thread?

Question

Using Thread is pretty straightforward

 Thread thread = new Thread(MethodWhichRequiresSTA);
 thread.SetApartmentState(ApartmentState.STA);  

How to accomplish the same using Tasks in a WPF application? Here is some code:

Task.Factory.StartNew
  (
    () => 
    {return "some Text";}
  )
   .ContinueWith(r => AddControlsToGrid(r.Result));  

I'm getting an InvalidOperationException with

The calling thread must be STA, because many UI components require this.

Solution

You can use the TaskScheduler.FromCurrentSynchronizationContext Method to get a TaskScheduler for the current synchronization context (which is the WPF dispatcher when you're running a WPF application).

Then use the ContinueWith overload that accepts a TaskScheduler:

var scheduler = TaskScheduler.FromCurrentSynchronizationContext();

Task.Factory.StartNew(...)
            .ContinueWith(r => AddControlsToGrid(r.Result), scheduler);

Comments

[Mrchief]

For any future visitors who come looking for the real intent of the question: - Using StaTaskScheduler (guide) as pointed in this answer - DIY version: stackoverflow.com/questions/16720496/…

[dudeNumber4]

Just to be clear; only the lambda within the ContinueWith method will run with the proper context, not what runs in the main task lambda.

[tina Miller]

Thanks for this, I was trying TaskScheduler.Current (from within a WinForms button event handler), couldn't understand why it wasn't working...

[Josh Sutterfield]

As a point of clarification, there is also an overload for the instance method Task.Start() which takes a TaskScheduler. The details of the question make it clear we're more interested in the continuation case, but the more general question of how to run a Task on an STA thread is not limited to only that case. I wrongly assumed at first I would need continue from an empty "fudge" task in order to run my desired STA task.

[Mrchief]

So shouldn't this question's title be edited to better reflect its intention?

[David]

Or broken down into two. "How do i start a task on an STA thread" and "How do i continue a task on an STA thread".

[Juan Pablo Garcia Coello]

public static Task<string> ShowOpenFolderPicker() { var scheduler = TaskScheduler.FromCurrentSynchronizationContext(); var tokensource = new CancellationTokenSource(); return Task.Factory.StartNew(Show,tokensource.Token, TaskCreationOptions.None,scheduler); }

[Juan Pablo Garcia Coello]

where for instance: private static String Show() { var dialog = new System.Windows.Forms.FolderBrowserDialog(); dialog.ShowNewFolderButton = true; var result = dialog.ShowDialog(); if (result == System.Windows.Forms.DialogResult.OK) { return dialog.SelectedPath; } return null; }

[Theodor Zoulias]

The ContinueWith and the Dispatcher are considered old-school approaches after the advent of async/await. I would avoid using them for new development (in general).