protected override void OnPreviewDragOver(RadDragOverEventArgs e)
        {
            TreeNodeElement targetNodeElement = e.HitTarget as TreeNodeElement;
            RadTreeViewElement treeView = e.HitTarget as RadTreeViewElement;

            if (treeView != null)
            {
                e.CanDrop = treeView.ElementTree.Control.AllowDrop && treeView.Nodes.Count == 0 && this.IsInValidState(treeView);
            }
            else if (targetNodeElement != null)
            {
                DropPosition dropPosition = this.GetDropPosition(this.DropLocation, targetNodeElement);
                e.CanDrop = this.CanDragOver(dropPosition, targetNodeElement);

                RadTreeViewDragCancelEventArgs args = new RadTreeViewDragCancelEventArgs(draggedNode, targetNodeElement.Data);
                args.Cancel = !e.CanDrop;
                args.DropPosition = dropPosition;

                this.owner.OnDragOverNode(args);
                e.CanDrop = !args.Cancel;                
            }

            base.OnPreviewDragOver(e);
        }

            if (dropPosition == DropPosition.AsChildNode && !targetNode.Expanded && targetNode.HasNodes)
            {
                targetTreeView.AutoExpand(targetNode);
                this.HideHintWindow();
                this.ShowHintWindow();
            }

        protected virtual internal void AutoExpand(RadTreeNode node)
        {
            if (!node.Expanded)
            {
                this.expandTimer.Tag = node;
                this.expandTimer.Start();
            }
        }