The bidirectional data link with the converter does not update the source


I've got a data binding set up with a converter to transform an awkward XML source to a display- and editing- convenient tree of internal classes. Everything works great for reading from the XML source, but I'm having a devil of a time trying to get changes made to the internal classes to propagate back to the XML source.

Here's the XAML for the use site:

        <local:SampleConverter x:Key="SampleConverter" />
        <Expander Header="Sample" >
                Sample="{Binding Path=XmlSource,
                                 Converter={StaticResource SampleConverter},
                                 Mode=TwoWay}" />

XmlSource is a CLR read-write property (not DependencyProperty) of the parent data bound object. It is a .NET type generated from an XSD.

SampleConverter implements IValueConverter. The Convert method is called and returns non-null data, but the ConvertBack method is never called.

SampleControl is a UserControl that encapsulates UI interaction with the Sample data tree. It's XAML looks like this:

<UserControl x:Class="SampleControl">
    [... other stuff ...]

        <Binding Path="Sample" RelativeSource="{RelativeSource Mode=Self}" Mode="TwoWay" TargetNullValue="{StaticResource EmptySampleText}" />

        <local:BoxedItemTemplateSelector />

The Sample property is a DependencyProperty in the SampleControl code behind:

public static readonly DependencyProperty SampleProperty =
    DependencyProperty.Register("Sample", typeof(SampleType), typeof(SampleControl), new PropertyMetadata(new PropertyChangedCallback(OnSampleChanged)));

public SampleType Sample
    get { return (SampleType)GetValue(SampleProperty); }
    set { SetValue(SampleProperty, value); }

private static void OnSampleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    if (e.NewValue != null)
        ((INotifyPropertyChanged)e.NewValue).PropertyChanged += ((SampleControl)d).MyPropertyChanged;
    else if (e.OldValue != null)
        ((INotifyPropertyChanged)e.OldValue).PropertyChanged -= ((SampleControl)d).MyPropertyChanged;

private void MyPropertyChanged(object sender, PropertyChangedEventArgs e)
    ;  // breakpoint here shows change notices are happening

The internal classes that the XmlSource is converted to implement INotifyPropertyChanged, and are sending change notifications up the tree, as indicated by a breakpoint in MyPropertyChanged above.

So if the data is reporting that it has changed, why isn't WPF calling my converter's ConvertBack method?

With hints from several similar questions and almost answers here on SO, I have a working solution that preserves the binding. You can manually force the binding to update the source in a strategically placed event, such as LostFocus:

private void mycontrol_LostFocus(object sender, RoutedEventArgs e)
    if (mycontrol.IsModified)
        var binding = mycontrol.GetBindingExpression(MyControl.SampleProperty);