TaskScheduler.FromCurrentSynchronizationContext block ui

Question

I have this xaml

<Window x:Class="TestCloseWindow.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Width="500" Height="400">
<StackPanel>
    <TextBlock x:Name="Seconds"></TextBlock>
    <Button Content="fasdfd" Click="ButtonBase_OnClick"></Button>
</StackPanel>
</Window>

And this code

public partial class MainWindow
{
    public MainWindow()
    {
        InitializeComponent();
    }
    private async void ButtonBase_OnClick(object sender, RoutedEventArgs e)
    {
         await CountToTen();
    }

    private Task CountToTen()
    {
       return Task.Factory.StartNew
              (() =>
                  {
                      for (var i = 1; i <= 10; i++)
                      {
                        Seconds.Text = i.ToString(CultureInfo.InvariantCulture);
                        Task.Delay(1000).Wait();
                      }
                   }
                   , CancellationToken.None
                   , TaskCreationOptions.None
                   , TaskScheduler.FromCurrentSynchronizationContext()
              );
    }
}

In this code I use TaskScheduler.FromCurrentSynchronizationContext() in order to access UI from background Task.
I expected that I can see how program count to ten, but instead of it I see blocked UI for 10 seconds and after 10 in TextBlock

How can I fix it?

chue x · Accepted Answer · 2013-03-28 14:00:58Z

2

You are using a blocking call Wait in your CountToTen function. To fix this you will need to use await instead. Which requires some other changes as well.

async private Task CountToTen()
{
    await Task.Factory.StartNew( async () =>
        {
            for (var i = 1; i <= 10; i++)
            {
                Seconds.Text = i.ToString(CultureInfo.InvariantCulture);
                //Task.Delay(1000).Wait();
                await Task.Delay(1000);
            }
        }, CancellationToken.None, TaskCreationOptions.None, TaskScheduler.FromCurrentSynchronizationContext());
}

edited Mar 28, 2013 at 14:00

answered Mar 28, 2013 at 13:39

chue x

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4 Comments

takayoshi Over a year ago

Got compile error. But it doesn't matter when I use this code without TaskScheduler.FromCurrentSynchronizationContext() (and delete Seconds.Text = ...) all works good

takayoshi Over a year ago

Error 2 The 'await' operator can only be used within an async lambda expression. Consider marking this lambda expression with the 'async' modifier. D:\TestProjects\TestCloseWindow\TestCloseWindow\MainWindow.xaml.cs 33 21 TestCloseWindow

chue x Over a year ago

@takayoshi - sorry. take a look now.

Servy Over a year ago

@chuex There's no need for the new task at all; it's just unneeded code bloat.

Servy · Accepted Answer · 2013-04-09 15:59:18Z

By using StartNew with TaskScheduler.FromCurrentSynchronizationContext(), in this particular case, you're not really doing anything at all. Since the entire task will be run in the UI thread it's no different from just executing it right there in line.

The primary source of your problem is Task.Delay(1000).Wait();. You're doing a blocking wait for a second, and you're doing that from the UI thread. You can refactor the code to not do a blocking wait, and also remove the needless task creation. This is all you need:

private async void ButtonBase_OnClick(object sender, RoutedEventArgs e)
{
    for (var i = 1; i <= 10; i++)
    {
        Seconds.Text = i.ToString(CultureInfo.InvariantCulture);
        await Task.Delay(1000)
    }
}

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