WPF DependencyProperty throws InvalidOperationException

Question

I am trying to set a dependency property which is updated by a WCF callback thread.
There is a ProgressBar on MainWindow.xaml that is bound to this property:

MainWindow.xaml

<ProgressBar Name="ProgressBar" Value="{Binding Progress, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" />

MainWindow has an instance of DemoModule, which is defined as:

DemoModule.xaml.cs

/// <summary>
///     Interaction logic for DemoModule.xaml
/// </summary>
public partial class DemoModule : UserControl, INotifyPropertyChanged
{
    public static readonly DependencyProperty ProgressProperty = DependencyProperty.Register("Progress", typeof(int), typeof(DemoModule));        
    public event ProgressEventHandler ProgressChanged;
    public event PropertyChangedEventHandler PropertyChanged;

    public int Progress
    {
        get { return (int)GetValue(ProgressProperty); }
        set { SetValue(ProgressProperty, value); }  // setter throws InvalidOperationException "The calling thread cannot access this object because a different thread owns it"
    }

    /// <summary>
    ///     Initializes a new instance of the <see cref="DemoModule" /> class.
    /// </summary>
    public DemoModule()
    {
        InitializeComponent();
        ProgressChanged += OnProgressChanged;
    }

    /// <summary>
    /// Called when [progress changed].
    /// </summary>
    /// <param name="sender">The sender.</param>
    /// <param name="args">The <see cref="ProgressChangedEventArgs" /> instance containing the event data.</param>
    public void OnProgressChanged(object sender, ProgressChangedEventArgs args)
    {
        Debug.WriteLine("Current Thread: {0}", Thread.CurrentThread.ManagedThreadId);
        Debug.WriteLine("Current Dispatcher Thread: {0}", Application.Current.Dispatcher.Thread.ManagedThreadId);

        if (ProgressChanged == null) return;

        Debug.WriteLine("ProgressChangedEventArgs.Current: " + args.Current);
        Progress = Convert.ToInt32(args.Current * 100);
        OnPropertyChanged("Progress");
    }

    /// <summary>
    /// Called when [property changed].
    /// </summary>
    /// <param name="propertyName">Name of the property.</param>
    [NotifyPropertyChangedInvocator]
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        Trace.WriteLine("Property " + propertyName + " changed. Value = " + Progress);
        var handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }

}

The Progress.set() is throwing an exception because of the thread affinity.
How can I fix this?

Update 1

This is allegedly thread safe, but has no effect:

public int Progress
{
    get
    {
        return Dispatcher.Invoke((() => (int)GetValue(ProgressProperty))); 
    }
    set
    {
        Dispatcher.BeginInvoke((Action)(() => SetValue(ProgressProperty, value)));
    }
 }

Update 2

My DemoModule.xaml.cs has a reference to a client library which implements the WCF callback method OnUpdateProgress:

InstallerAgentServiceClient.cs

public void OnUpdateProgress(double progress)
        {
            //Debug.WriteLine("Progress: " + progress*100 + "%");
            var args = new ProgressChangedEventArgs(progress, 1, "Installing");
            _installModule.OnProgressChanged(this, args);
        }

The _installModule object above is the instance of DemoModule.

Update 3

After removing the [CallBackBehavior] attribute from the WCF client library, there no longer seems to be thread synchronization issues. I can update the progress bar in the MainWindow as follows:

DemoModule.xaml.cs

public void OnProgressChanged(object sender, ProgressChangedEventArgs args)
{
    Progress = Convert.ToInt32(args.Current * 100);
    var progressBar = Application.Current.MainWindow.FindName("ProgressBar") as ProgressBar;
    if (progressBar != null)
        progressBar.Value = Progress;
}

Answer 1

I recommand using the IProgress interface. Works like a charm for me and is pretty easy to use. In your progressbarVM add

public double Actualprogress
{
get { return (double)GetValue(ActualprogressProperty); }
set { SetValue(ActualprogressProperty, value); }
}
public static readonly DependencyProperty ActualprogressProperty =
DependencyProperty.Register("Actualprogress", typeof(double), typeof(ProgressBar), 
new PropertyMetadata(0.0));

then call your method as an asyn task using await like so :

var progress = new Progress<double>(progressPercent => 
progressBarVM.progressBar.Actualprogress = progressPercent);
Parser parser = new Parser();
ModelVMResult result = await Task.Run(() => parser.Parse(filename,progress));

then in your method "parse" just do:

float threshold = 0.0f;
for (int i = 0; i < count; i++)
{
if (i >= threshold)
{ progress.Report(prog += 1); threshold += count / 100.0f; }
this.Readline(reader, i);
}

Of course you need to bind your xaml progressbar.Value to ProgressbarVM.Actualprogress. Then your progressbar will update and your app will still be responsive during the process.

Answer 2

You need to update your DepedencyProperty via the UI Thread. Use:

Application.Current.Dispatcher.BeginInvoke(Action)

Or:

Application.Current.Dispatcher.Invoke(Action)