Question

I am writing a Windows Store App toy application for Windows 8.
It has just one xaml page with a TextBlock. The page has the class MyTimer as DataContext :

this.DataContext = new MyTimer();

MyTimer implements INotifyPropertyChanged and the updating of the property Time is made with a timer:

public MyTimer(){
    TimerElapsedHandler f = new TimerElapsedHandler(NotifyTimeChanged);
    TimeSpan period = new TimeSpan(0, 0, 1);
    ThreadPoolTimer.CreatePeriodicTimer(f, period);
}

with

private void NotifyTimeChanged(){
    if (this.PropertyChanged != null){
        this.PropertyChanged(this, new PropertyChangedEventArgs("Time"));
    }
}

the TextBlock has a databinding on Time

<TextBlock Text="{Binding Time}" />

When I run the application i have the following exception:

System.Runtime.InteropServices.COMException was unhandled by user code

With the message

The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))

The real problem is that I am updating the property of the class MyTimer, not the GUI itself,
I can’t figure it out, but I think the solution should use something like this one.

Answer 1

Yes, you’re notifying property changes from a thread pool thread rather than the UI thread. You need to marshal the notification back to the UI thread in the timer callback. Now, your view model is separated from your view (a good thing) therefore it doesn’t have a direct link to the Dispatcher infrastructure. So what you want to do is hand it the proper SynchronizationContext on which to communicate. To do this you need to capture the current SynchronizationContext during construction or allow it to be passed in explicitly to a constructor which is good for tests or if you’re initializing the object off the UI thread to begin with.

The whole shebang would look something like this:

public class MyTimer
{
    private SynchronizationContext synchronizationContext;

    public MyTimer() : this(SynchronizationContext.Current)
    {
    }

    public MyTimer(SynchronizationContext synchronizationContext)
    {
        if(this.synchronizationContext == null)
        {
            throw new ArgumentNullException("No synchronization context was specified and no default synchronization context was found.")
        }

        TimerElapsedHandler f = new TimerElapsedHandler(NotifyTimeChanged);
        TimeSpan period = new TimeSpan(0, 0, 1);
        ThreadPoolTimer.CreatePeriodicTimer(f, period);
    }

    private void NotifyTimeChanged()
    {
        if(this.PropertyChanged != null)
        {
            this.synchronizationContext.Post(() =>
                {
                    this.PropertyChanged(this, new PropertyChangedEventArgs("Time"));
                });
        }
    }
}