C# mixed use of Task and Dispatcher.Invoke, why it halts?

Question

I tried below snippet:

    public Task RunUiTask(Action action)
    {
        var task = Task.Factory.StartNew(() =>
        {
            Dispatcher.Invoke(DispatcherPriority.Background, action);
        });
        return task;
    }

    private void OnCreateTask(object sender, RoutedEventArgs e)
    {
        var task = RunUiTask(() =>
            {
                for(int i=0;i<10;i++)
                {
                    ResultTextBlock.Text += i.ToString();
                }
            });
        task.Wait();                        //(a) Program stopped here
        ResultTextBlock.Text += "Finished"; //(b) Never called;
    }

I couldn't understand why, when OnCreateTask (a button click event handler) is called, the program halts at (a), and (b) is never called.

Note: I know I can use Dispatcher.BeginInvoke to make program responsive, but this is not my concern here.

Can any body tell why the program halts at (a), and why (b) is never called? Thanks.

Answer 1

The call of

Dispatcher.Invoke(DispatcherPriority.Background, action);

will execute Action in your UI Thread and will return after Action is executed. The Problem is, that your UI Thead is blocked because of the task.Wait() in your OnCreateTask, so the Action will never be executed and you have a Deadlock.

EDIT

Instead of your task.Wait() you should use a Continuation and Update ResultTextBlock.Text

task.ContinueWith(t=>{
     ResultTextBlock.Text += "Finished";
}, TaskScheduler.FromCurrentSynchronizationContext());