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

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);
    }
}