Calling a delegate on the UI thread from a work thread inside a child class.

Question

Hi All,
I've run into a snag developing a WPF multithreaded app where I need to call a method on the UI thread from my work thread, where the work thread is running a different class.
Currently I am trying to use a delegate and an event in the 2nd class to call a method in the 1st class on the UI thread. This so far is not working as because the 2nd class is running in its own thread, it causes a runtime error when attempting to call
the event.
I've seen lots of solutions referring to using the dispatcher to solve this by invoking the code, however my work thread is running a different class than my UI thread, so it seems the dispatcher is not available?
Below is as simplified an example as I can make of what I am trying to achieve. Currently the below code results in a "The calling thread cannot access this object because a different thread owns it." exception at runtime.
The XAML side of this just produces a button connected to startThread2_Click() and a label which is then intended to be updated by the 2nd thread calling the updateLabelThreaded() function in the first thread when the button is clicked.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Windows.Threading;
using System.Threading;
namespace multithreadtest
public delegate void runInParent();
public partial class MainWindow : Window
public MainWindow()
InitializeComponent();
threadUpdateLabel.Content = "Thread 1";
private void startThread2_Click(object sender, RoutedEventArgs e)
thread2Class _Thread2 = new thread2Class();
_Thread2.runInParentEvent += new runInParent(updateLabelThreaded);
Thread thread = new Thread(new ThreadStart(_Thread2.threadedTestFunction));
thread.Start();
public void updateLabelThreaded()
threadUpdateLabel.Content = "Thread 2 called me!";
public class thread2Class
public event runInParent runInParentEvent;
public void threadedTestFunction()
if (runInParentEvent != null)
runInParentEvent();
I'm unfortunately not very experienced with c# so I may well be going the complete wrong way about what I'm trying to do. In the larger application I am writing, fundamentally I just need to be able to call a codeblock in the UI thread when I'm in a different
class on another thread (I am updating many different items on the UI thread when the work thread has performed certain steps, so ideally I want to keep as much UI code as possible out of the work thread. The work threads logic is also rather complicated as
I am working with both a webAPI and a MySQL server, so keeping it all in its own class would be ideal)
If a more thorough explanation of what I am trying to achieve would help please let me know.
Any help with either solving the above problem, or suggestions for alternative ways I could get the class in the UI thread to do something when prompted by the 2nd class in the 2nd thread would be appreciated.
Thanks :)

Answer 1

If I follow the explanation, I think you can use MVVM Light messenger.
You can install it using NuGet.
Search on mvvm light libraries only.
You right click solution in solution explorer and choose manage nugget...
So long as you're not accessing ui stuff on these other threads.
using GalaSoft.MvvmLight.Messaging;
namespace wpf_Tester
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
public MainWindow()
InitializeComponent();
Messenger.Default.Register<String>(this, (action) => ReceiveString(action));
private void ReceiveString(string msg)
MessageBox.Show(msg);
Dispatcher.BeginInvoke((Action)delegate()
tb.Text = msg;
private void Button_Click(object sender, RoutedEventArgs e)
Task.Factory.StartNew(() => {
Messenger.Default.Send<String>("Hello World");
What the above does is start up a new thread - that startnew does that.
It sends of message of type string which the main window has subscribed to....it gets that and puts up a message box.
The message is sent from the window to the window in that but this will work across any classes in a solution.
Note that the receive acts on whichever thread the message is sent from.
I would usually be altering properties of a viewmodel with such code which have no thread affinity.
If you're then going to directly access properties of ui elements then you need to use Dispatcher.BeginInvoke to get back to the UI thread.
I do that with an anonymous action in that bit of code but you can call a method or whatever instead if your logic is more complicated or you need it to be re-usable.
http://social.technet.microsoft.com/wiki/contents/articles/26070.aspx
Hope that helps.
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