public class XComboBox : ComboBox
    {
        Popup _popup = null;
        ToggleButton _toggle = null;
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            _popup = GetTemplateChild("PART_Popup") as Popup;
            _toggle = this.GetChildOfType<ToggleButton>();
            _toggle.SetBinding(ToggleButton.IsCheckedProperty, new Binding("IsDropDownOpenX") { Source = this, Mode = BindingMode.TwoWay });
            _popup.SetBinding(Popup.IsOpenProperty, new Binding("IsChecked") { Source = _toggle, Mode = BindingMode.OneWay });
        }
        public static readonly DependencyProperty IsDropDownOpenXProperty = DependencyProperty.Register(
            "IsDropDownOpenX", typeof(bool), typeof(XComboBox), new UIPropertyMetadata(false, (s, e) => {
                XComboBox xcb = (XComboBox)s;
                xcb.IsDropDownOpen = xcb.IsDropDownOpenX;
            }));
        public bool IsDropDownOpenX { get { return (bool)this.GetValue(IsDropDownOpenXProperty); } set { this.SetValue(IsDropDownOpenXProperty, value); } }
    }
    public static class ExtensionMethods
    {
        public static T GetChildOfType<T>(this DependencyObject depObj)
            where T : DependencyObject
        {
            if (depObj == null) return null;

            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++) {
                var child = VisualTreeHelper.GetChild(depObj, i);

                var result = (child as T) ?? GetChildOfType<T>(child);
                if (result != null) return result;
            }
            return null;
        }
    }