How can I alter XAML elements from a backgroundworker thread?

Question

In the following code example, I want to change the color of the Foreground text of a TextBox from my BackgroundThread, but it gets the error:

This thread cannot access this object since it is in another thread.

What do I have to change in the code below so that the background worker thread can change the foreground color in the TextBox?

Answer:

Thanks Andy, it was just that small oversight, here is the corrected code for posterity's sake:

using System.Windows;
using System.ComponentModel;
using System.Threading;
using System.Windows.Media;

namespace TestBackgroundWorker7338
{
    public partial class Window1 : Window
    {
        private BackgroundWorker _worker;
        int _percentageFinished = 0;

        public Window1()
        {
            InitializeComponent();
            ButtonCancel.IsEnabled = false;
        }

        private void Button_Start(object sender, RoutedEventArgs e)
        {
            _worker = new BackgroundWorker();
            _worker.WorkerReportsProgress = true;
            _worker.WorkerSupportsCancellation = true;

            _worker.DoWork += (s, args) =>
            {
                BackgroundWorker worker = s as BackgroundWorker;
                int numberOfTasks = 300;
                for (int i = 1; i <= numberOfTasks; i++)
                {
                    if (worker.CancellationPending)
                    {
                        args.Cancel = true;
                        return;
                    }

                    Thread.Sleep(10);
                    float percentageDone = (i / (float)numberOfTasks) * 100f;
                    worker.ReportProgress((int)percentageDone);
                }
            };

            _worker.ProgressChanged += (s,args) =>
            {
                _percentageFinished = args.ProgressPercentage;
                ProgressBar.Value = _percentageFinished;
                Message.Text = _percentageFinished + "% finished";
                if (_percentageFinished < 500)
                {
                    Message.Text = "stopped at " + _percentageFinished + "%";
                }
                else
                {
                    Message.Text = "finished";
                }

                if (_percentageFinished >= 70)
                {
                    InputBox.Foreground = new SolidColorBrush(Colors.Red);
                }
                else if (_percentageFinished >= 40)
                {
                    InputBox.Foreground = new SolidColorBrush(Colors.Orange);
                }
                else if (_percentageFinished >= 10)
                {
                    InputBox.Foreground = new SolidColorBrush(Colors.Brown);
                }
                else
                {
                    InputBox.Foreground = new SolidColorBrush(Colors.Black);
                }

            };

            _worker.RunWorkerCompleted += (s,args) =>
            {
                ButtonStart.IsEnabled = true;
                ButtonCancel.IsEnabled = false;
                ProgressBar.Value = 0;
            };

            _worker.RunWorkerAsync();
            ButtonStart.IsEnabled = false;
            ButtonCancel.IsEnabled = true;

        }

        private void Button_Cancel(object sender, RoutedEventArgs e)
        {
            _worker.CancelAsync();
        }
    }
}

Answer 1

The ProgressChanged delegate is run on the UI thread. If you set the BackgroundColor there instead of in DoWork, that should allow you to set the color without the error.

Answer 2

As Andy said changing properties of controls must happen on the UI thread.

WPF provides a Dispatcher class that makes it easier to route calls for controls to the appropriate UI thread. Controls in WPF exposes a Dispatcher property object to dispatch the call to an appropriate UI thread.

You can use this as follows (I would add this after the line worker.ReportProgress((int)percentageDone); in the Button_Start method):

// the Window class exposes a Dispatcher property, alternatively you could use
// InputBox.Dispatcher to the same effect
if (!Dispatcher.CheckAccess())
     {
        Dispatcher.Invoke(new Action(() => ChangeForegroundColor(percentageDone));
     }
     else
     {
        ChangeForegroundColor(percentageDone);
     }

...
private void ChangeForegroundColor(int percentageDone)
{
    if (percentageDone >= 90)
    {
        InputBox.Foreground = new SolidColorBrush(Colors.Red);
    }
    else if(percentageDone >=10)
    {
        InputBox.Foreground = new SolidColorBrush(Colors.Orange);
    }
}