public TreeViewItemAdorner(UIElement adornedElement) : base(adornedElement) { var treeViewItem = adornedElement.ParentsUntil<TreeViewItem>(); var parentTreeViewItem = treeViewItem.ParentsUntil<TreeViewItem>(); _parentAdorner = parentTreeViewItem?.FindBehaviorInChildren<TreeViewItemDroppableBehavior>().Adorner; _isParentFirstElement = parentTreeViewItem?.Items[0] == treeViewItem.DataContext; IsHitTestVisible = false; BorderBrush = Helpers.FindResource<SolidColorBrush>("DroppableAdornerBorderBrush"); BackgroundBrush = Helpers.FindResource<SolidColorBrush>("DroppableAdornerBackgroundBrush"); LineBrush = Helpers.FindResource<SolidColorBrush>("DroppableAdornerLineBrush"); }