Unintentional Multi-threading in Windows Runtime

I am working with a SettingsPane that holds settings, and I have all my problems solved - except for one thing I apparently have no control over.

This is the relevant SettingsPane.xaml:

<ComboBox Name="Themes" SelectionChanged="SettingSelectionChanged">
    <ComboBoxItem Content="Theme 1" />
    <ComboBoxItem Content="Theme 2" />
    <ComboBoxItem Content="Theme 3" />
</ComboBox>

This is my SettingsPane.xaml.cs:

public Settings()
{
    this.InitializeComponent();
    this.DataContext = MainPage.Data;
    Themes.SelectedIndex = 0;
}

private void SettingSelectionChanged(object sender, SelectionChangedEventArgs e)
{
    ApplicationData.Current.RoamingSettings.Values["Theme"] = Themes.SelectedIndex;
    ApplicationData.Current.SignalDataChanged();
}

I handle the SignalDataChanged() call in MainPage.xaml.cs:

public static MainPageVM Data = new MainPageVM();

public MainPage()
{
    Windows.Storage.ApplicationData.Current.DataChanged += (a, o) =>
    {
        Data.Theme = (int) Windows.Storage.ApplicationData.Current.RoamingSettings.Values["Theme"];
    };
}

Theme is stored in MainPageVM.cs:

private int _theme = 0;
public int Theme
{
    get { return _theme; }
    set
    {
        if (value == _theme) return;
        _theme = value;
        OnPropertyChanged();
    }
}

Now, this is bound like this:

<Grid Background="{Binding Theme, Converter={StaticResource ThemeToBackground}}" Name="MainGrid">

It appears to be working, except for one thing. When it hits the OnPropertyChanged() call in Theme's setter, it crashes with this error:

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

What can I do about this?

Each window has a different thread associated with it; you should avoid referencing one page's view model from another page if there is data binding involved (you will need an adapter that pushes the changes onto the correct thread). [Edit] Also that particular event is being raised on a thread that isn't associated with any page.