Binding PropertyGridPropertyItem's Value to multiple enumerators

Hello,

How to bind in code behind a PropertyGridPropertyItem's Value to a HorizontalListBox's SelectedItems property where the HorizontalListBox data template has SelectionMode set to Multiple.

Basically I want a property like the one displayed in the "SampleBrowser\ProductSamples\PropertyGridSamples\QuickStart\CustomPropertyEditor" demo, but with the ability to have multiple selection for the property's value.

Among numerous trial and error attempts, I've tried this too, but it throws an exception because the SelectedItemsProperty has no setter through binding.

_propertyGrid = new PropertyGrid();

SnapToPanels = new PropertyGridPropertyItem
            {
                //ValueType = HorizontalListBox.SelectedItemsProperty.GetType(),
                ValueType = typeof(List<OnOffAuto>),
                //Value = OnOffAuto.Off,
                //DefaultValue = OnOffAuto.On,
            };

var dataTemplate = new DataTemplate();
var elementFactory = new FrameworkElementFactory(typeof(MyHorizontalListBox));

var binding2 = new Binding
            {
                Source = elementFactory,
                Path = new PropertyPath(path: MyHorizontalListBox.SelectedItemsProperty.ToString()),
                Mode = BindingMode.TwoWay,
            };
            SnapToPanels.SetBinding(PropertyGridPropertyItem.ValueProperty, binding: binding2);

            dataTemplate.VisualTree = elementFactory;

            SnapToPanels.ValueTemplate = dataTemplate;

internal class MyHorizontalListBox : HorizontalListBox
{
public MyHorizontalListBox()
{
this.SelectionMode = System.Windows.Controls.SelectionMode.Multiple;

Items.Add(OnOffAuto.Auto);
Items.Add(OnOffAuto.Off);
Items.Add(OnOffAuto.On);

}
}

   public enum OnOffAuto
        {
            /// <summary>
            /// Automatic.
            /// </summary>
            Auto,

            /// <summary>
            /// On.
            /// </summary>
            On,

            /// <summary>
            /// Off.
            /// </summary>
            Off
        }

 Please help, thank you.