Making sure OnPropertyChanged() is called on UI thread in MVVM WPF app

Question

In a WPF app that I'm writing using the MVVM pattern, I have a background process that doing it's thing, but need to get status updates from it out to the UI.

I'm using the MVVM pattern, so my ViewModel knows virtually nothing of the view (UI) that is presenting the model to the user.

Say I have the following method in my ViewModel:

public void backgroundWorker_ReportProgress(object sender, ReportProgressArgs e)
{
    this.Messages.Add(e.Message);
    OnPropertyChanged("Messages");
}

In my view, I have a ListBox bound to the Messages property (a List<string>) of the ViewModel. OnPropertyChanged fulfills the role of the INotifyPropertyChanged interface by calling a PropertyChangedEventHandler.

I need to ensure that OnPropertyChanged is called on the UI thread - how do I do this? I've tried the following:

public Dispatcher Dispatcher { get; set; }
public MyViewModel()
{ 
    this.Dispatcher = Dispatcher.CurrentDispatcher;
}

Then adding the following to the OnPropertyChanged method:

if (this.Dispatcher != Dispatcher.CurrentDispatcher)
{
    this.Dispatcher.Invoke(DispatcherPriority.Normal, new ThreadStart(delegate
    {
        OnPropertyChanged(propertyName);
    }));
    return;
}

but this did not work. Any ideas?

Answer 1

WPF automatically marshals property changes to the UI thread. However, it does not marshal collection changes, so I suspect your adding a message is causing the failure.

You can marshal the add manually yourself (see example below), or use something like this technique I blogged about a while back.

Manually marshalling:

public void backgroundWorker_ReportProgress(object sender, ReportProgressArgs e)
{
    Dispatcher.Invoke(new Action<string>(AddMessage), e.Message);
    OnPropertyChanged("Messages");
}

private void AddMessage(string message)
{
    Dispatcher.VerifyAccess();
    Messages.Add(message);
}

Answer 2

I REALLY like Jeremy's answer: Dispatching In Silverlight

Summary:

• Placing Dispatcher in ViewModel seems inelegant

• Creating an Action<Action> property, set it to just run the action in the VM constructor

• When using the VM from the V, set the Action property to invoke the Dispatcher