WPF TabControl - cannot access this object because a different thread owns it

Question

I'm developing a WPF Login form.I have a tab control with two tabs:

tab1) Contains the inputs for login(user name and password text box/labels)

tab2) Contains a custom animation which is used as the progress bar

Once the user captures all the info and clicks Login in the Login button's click event I set the active tab to tab2 and the progress bar is shown to the user. If an error occurs during this step I would like to return the user to tab1 and this is where I get the following error:

Invalid Operation Exception (The calling thread cannot access this object because a different thread owns it.)

Please advice how I can kill the thread or any other work around to help fix my problem

My code:

public partial class LogonVM : ILogonVM
{
    private IWebService _webService;
    private static TabControl loaderTabs;

    private string userName = String.Empty;
    public string UserName
    {
        get { return userName; }
        set
        {
            userName = value;
            OnPropertyChanged("UserName", true);
        }
    }

    private SecureString password = new SecureString();
    public SecureString Password
    {
        get { return password; }
        set
        {
            password = value;
            OnPropertyChanged("Password", true);
        }
    }

    public MinimalLogonViewModel(MinimalLogonView view,IWebService webService)
    {
            _webService = webService;

            View = view;
            view.DataContext = this;

            loaderTabs = (TabControl)this.View.FindName("loaderTabs");
        }

        catch (Exception eX)
        {
            MessageBox.Show(eX.Message);
        }
    }

    protected virtual void OnPropertyChanged(string propertyName, bool raiseCanExecute)
    {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));

            if (raiseCanExecute)
                LogonCommand.RaiseCanExecuteChanged();
    }

    private void Logon(object parameter)
    {
        SetActiveTab(TabType.Loader);

        _messageBroker.onAuthenticated += new EventHandler(_MessageBroker_onAuthenticated);
        Task.Execute((DispatcherWrapper)View.Dispatcher,
                     () => _webService.Authenticate(userName, password.ConvertToUnsecureString()),
                     (ex) =>
                     {
                         if (ex != null)
                         {                       
                             //This is where I'm having issues
                             //If an error occurs I want to switch back to the Login tab which will enable the user to try Login again
                             //This does not throw an error but it also doesn't show the Login tab
                             SetActiveTab(TabType.Login);
                         }
                         else
                         {
                            //No error perform additional processing
                         }
                     });
    }

    private void SetActiveTab(TabType type)
    {
        //If I leave the code as simply:
        //loaderTabs.SelectedIndex = (int)type;
        //I get an error when seting the tab for the second time:
        //Invalid Operation Exception (The calling thread cannot access this object because a different thread owns it.) 

        loaderTabs.Dispatcher.Invoke((Action)(() =>
        {
            loaderTabs.SelectedIndex = (int)type;
        }));
    }
}

rcreff rcreff · Accepted Answer · 2012-08-29T20:40:35

I'm no WPF expert but I'm wondering why you would use the dispatcher object for this functionality surely you could just do this?

private void SetActiveTab(TabType type) 
{
  loaderTabs.SelectedIndex = (int)type; 
}

EDIT:

Ok I fully understand now why you would use the dispatcher duh. I tried the bits on your code while processing on a seperate thread and it worked for me.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
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.Threading;
using System.ComponentModel;


namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
public partial class MainWindow : Window
{

    private BackgroundWorker _worker;


    public MainWindow()
    {
        InitializeComponent();
        _worker = new BackgroundWorker();
        _worker.DoWork += new DoWorkEventHandler(_worker_DoWork);
        _worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(_worker_RunWorkerCompleted);

    }

    void _worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        MessageBox.Show("Done");
    }

    void _worker_DoWork(object sender, DoWorkEventArgs e)
    {
        Thread.Sleep(5000);
    }

    private void SetActiveTab(TabType type)
    {
        loaderTabs.Dispatcher.Invoke((Action)(() =>
        {
            //This is where the error happens when I try set the active tab back to tab1 
            loaderTabs.SelectedIndex = (int)type;
        }));
    }


    public void Login(string userName, string password)
    {
        try
            {
               SetActiveTab(TabType.Loader);
               //Processing... 
               _worker.RunWorkerAsync();
           }
           catch (Exception)
           {
               SetActiveTab(TabType.Login);
           }
       }

        enum TabType {Login, Loader};

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Login("user", "password");
        }

   }
}

WPF TabControl - cannot access this object because a different thread owns it

2 Answers