why isn't possible to transfer ListA from Slave Thread to Main Thread?

Question

I'm trying to generate and dispatch (while having my UI Thread with progressRing.IsActive = true;), three List Objects on a BackgroundWorker and then transfer said list to the UI Thread, but I'm running into issues with...

Must create DependencySource on same Thread as the DependencyObject.

Resources, I've read

• C# lock (MSDN Documentation)
• Sending Arguments to Background Worker (Stackoverflow)
• C# Background Worker (MSDN Documentation)
• C# getting Background Worker to return a result (Stackoverflow)

Method BackgroundLogin() of Partial Class MainWindow

    private void BackgroundLogin()
    {
        //process the form on UI Thread
        this.LoginForm(FadeOut);
        this.LoadingRing(FadeIn);

        //Start a new Thread
        BackgroundWorker worker = new BackgroundWorker();
        worker.DoWork += worker_DoWork;
        worker.RunWorkerCompleted += worker_RunWorkerCompleted;
        //initialize custom class WorkerThread object to store/process a result
        WorkerThread wt = new WorkerThread(this, txtEmailAddress.Text, txtPassword.Password);
        //start the worker and send the object across.
        worker.RunWorkerAsync(wt);
    }

Method worker_DoWork of Partial Class MainWindow

    private void worker_DoWork(object sender, DoWorkEventArgs e)
    {
        //grab the object
        WorkerThread wt = (WorkerThread)e.Argument;
        //check if we can login
        if (!wt.Login())
        {
            //cancel the thread
            e.Cancel = true;
        }
        else
        {
            //load additional data
            wt.UpdateAuthLbl(".. Loading New Data ..");
            wt.LoadLists();
            wt.UpdateAuthLbl(".. Data Loaded ..");
        }
        //pass the object back
        e.Result = wt;
    }

Method loadLists() of Class WorkerThread

    /// <summary>
    /// Load data into the list
    /// </summary>
    public void LoadLists()
    {
        this.gene_fact_list = db.loadGeneFactTable();
        this.gene_trait_fact_list = db.loadGeneTraitFactTable(this.gene_fact_list);
        this.category_trait_list = db.loadCategoryTraits();
    }

Method worker_RunWorkerCompleted of Partial Class MainWindow, Object gl of Class GeneList

    private void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        //grab the finished object
        WorkerThread wt = (WorkerThread) e.Result;
        //work out if we are logged in
        Boolean LoginFlag = !e.Cancelled && e.Error == null;
        if (LoginFlag)
        {
            lblAuthentication.Content = ".. Loading Interface ..";
            //pass the finished object into memory
            this.gl = wt;
            //reset the listbox
            this.resetListBox();
        }
        this.LoginForm(LoginFlag);
    }

Method resetListBox() and ListBoxItems

    /// <summary>
    /// Load the data for the form
    /// </summary>
    public void resetListBox()
    {
        if (this.gl.notNullOrEmpty())
        {
            this.ListBoxItems.Clear();
            //begin compiling the mainTab
            foreach (KeyValuePair<long, GeneNotesDataModel> kvp in this.gl.gene_fact_list)
            {
                this.ListBoxItems.Add(kvp.Value);
            }
        }
    } //close function

    //declare WPF list binding
    private ObservableCollection<GeneNotesDataModel> _listBoxItems = new ObservableCollection<GeneNotesDataModel>();

    /// <summary>
    /// Control the listbox of rsid codes
    /// </summary>
    public ObservableCollection<GeneNotesDataModel> ListBoxItems
    {
        get { return _listBoxItems; }
        set { _listBoxItems = value; }
    }

XAML ListBox lstSnpCodes

<ListBox ItemsSource="{Binding ElementName=UI, Path=ListBoxItems}" Margin="6,38,0,60" BorderThickness="2" HorizontalAlignment="Left" Width="180" Name="lstSnpCodes" SelectionChanged="lstSnpCodes_SelectionChanged" KeyUp="OnKeyUpHandler" />

The line this.ListBoxItems.Add(kvp.Value); causes the Exception to occur (If I replace it with Debug.WriteLine(kvp.Value.getValueAsString()); it will run just fine). Any ideas on why I'm getting a DependencySource exception? and why isn't possible to transfer ListA from Slave Thread to Main Thread?

PasteBin Links expire in April 2013

Answer 1

Instead of trying to access the property directly across threads, try accessing the variable behind it.

In sample app:

private static List<object> _lst;


    static void Main(string[] args)
    {
        Task tsk =  new Task(() => _lst = new List<object>()); //task will create new thread if available and execute the Action.
        tsk.Start(); //initiates task, which initiates new List<object> on new thread.
        tsk.Wait(); //waits for task to complete.  Note that this locks main thread.
        _lst.Add(1); //Attempt to access _lst from main thread.
    }

Specific to your example.

private ReadOnly object _lockMyList = new object();
private List _MyList;

public void SetMyList(List lst) //Use this to set list from other thread.
{
    Lock(_lockMyList)
    {
        _MyList = lst;
    }
    //Invoke Property Changed event if needed for binding.
}

public List MyList
{
    get
    {
        List lst = null;
        Lock(_lockMyList)
        {
            lst = _MyList; //You might need to do _MyList.ToList() for true thread safety
        }
        return lst;
    }
    //Property setter if needed, but use accessor method instead for setting property from other thread
}

Also, you may want to look into SynchronizedCollection to see if it is more suitable for your needs.

I hope this helps.