Question

If the following code is run on the background thread, how can I ‘ContinueWith’ on the main thread?

  var task = Task.Factory.StartNew(() => Whatever());
  task.ContinueWith(NeedThisMethodToBeOnUiThread), TaskScheduler.FromCurrentSynchronizationContext())

The above will not work, because the current synchronization context is already a background thread.

Answer 1

You need to get a reference to TaskScheduler.FromCurrentSynchronizationContext() from the UI thread and pass it to the continuation.

Similar to this.
http://reedcopsey.com/2009/11/17/synchronizing-net-4-tasks-with-the-ui-thread/

private void Form1_Load(object sender, EventArgs e)
{
    // This requires a label titled "label1" on the form...
    // Get the UI thread's context
    var context = TaskScheduler.FromCurrentSynchronizationContext();

    this.label1.Text = "Starting task...";

    // Start a task - this runs on the background thread...
    Task task = Task.Factory.StartNew( () =>
        {
            // Do some fake work...
            double j = 100;
            Random rand = new Random();
            for (int i = 0; i < 10000000; ++i)
            {
                j *= rand.NextDouble();
            }

            // It's possible to start a task directly on
            // the UI thread, but not common...
            var token = Task.Factory.CancellationToken;
            Task.Factory.StartNew(() =>
            {
                this.label1.Text = "Task past first work section...";
            }, token, TaskCreationOptions.None, context);

            // Do a bit more work
            Thread.Sleep(1000);
        })
        // More commonly, we'll continue a task with a new task on
        // the UI thread, since this lets us update when our
        // "work" completes.
        .ContinueWith(_ => this.label1.Text = "Task Complete!", context);
}