can a worker thread read a control in the GUI?

Question

I got a thread running every few seconds fetching some data from a db but this is based on the selection on a listbox and on a few checkboxes... can I read the values of these controls without using the GUI thread?

The data is also read whenever one of the controls change, but the data might change in db without warning...hence the thread running every few seconds.

I'm working with wpf C#

Answer 1

Of course you can, and it is the way to do in case it takes some time to finish the work, so that you don't need to block the GUI thread.

Answer 2

In short, no. Controls can only be accessed by the thread that created them.

Prove it for yourself:

<Window x:Class="ReadControlsBackground.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow"
        Height="350"
        Width="525">
    <StackPanel>
        <Button x:Name="Start"
                Click="Button_Click"
                Content="Start" />
        <ListBox x:Name="List">
            <ListBoxItem>One</ListBoxItem>
            <ListBoxItem>Two</ListBoxItem>
        </ListBox>
        <CheckBox x:Name="Check" />
    </StackPanel>
</Window>



using System;
using System.Diagnostics;
using System.Threading;
using System.Windows;

namespace ReadControlsBackground {
    /// <summary>
    ///   Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window {
        public MainWindow() {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e) {
            Start.IsEnabled = false;
            var t = new Thread(Poll);
            t.Start();
        }

        private void Poll() {
            while (true) {
                Debug.WriteLine(String.Format("List: {0}", List.SelectedValue));
                Debug.WriteLine(String.Format("Check: {0}", Check.IsChecked));
                Thread.Sleep(5000);
            }
        }
    }
}

Do something like this instead:

private void Poll() {
    while (true) {
        var selected = (String) Dispatcher.Invoke((Func<String>) (() => (List.SelectedValue ?? "?").ToString()));
        var isChecked = (Boolean?) Dispatcher.Invoke((Func<Boolean?>) (() => Check.IsChecked));
        Debug.WriteLine(String.Format("List: {0}", selected));
        Debug.WriteLine(String.Format("Check: {0}", isChecked));
        Thread.Sleep(5000);
    }
}

Answer 3

Hello,

If you want to access the GUI from another thread. Then you will have to marshal the call to the GUI thread.

I don't have the exact details as I have not done C# for a very long time.

If I manage to find something I will write back with some code snippet.

Answer 4

Don't use a thread, use a timer. Then you're already in the UI context thread when the timer event fires.

Answer 5

Try something like this:

string textBoxData = string.Empty;

/* Blocks current thread until the UI
 * thread executes the anonymous method */
Dispatcher.Invoke((Action) delegate
{
    textBoxData = txtBox.Text;
});

/* Do more work */

You may need to change "Dispatcher.Invoke" to "someWpfControl.Dispatcher"

Answer 6

This problem becomes trivial if you do it the other way around. Have the UI thread read whatever values are required, then let it start the thread, passing those values. ThreadPool.QueueUserWorkItem is ideal for this.