void IDropTarget.DragOver(UIElement draggedElement, Point mousePosition)
        {
            ColumnManagerCell cell = draggedElement as ColumnManagerCell;

            if (cell != null)
            {
                HierarchicalGroupByControlNode hierarchicalGroupByControlNode = this.GetHierarchicalGroupByControlNodeFromColumnManagerCell(cell);

                // It may not be visible
                if (hierarchicalGroupByControlNode != null)
                {
                    hierarchicalGroupByControlNode.ShowFarDropMark(cell, mousePosition);
                }
            }
            else
            {
                HierarchicalGroupByItem hierarchicalGroupByItem = draggedElement as HierarchicalGroupByItem;
                if (hierarchicalGroupByItem == null)
                {
                    return;
                }

                HierarchicalGroupByControlNode hierarchicalGroupByControlNode = this.GetHierarchicalGroupByControlNodeFromHierarchicalGroupByItem(hierarchicalGroupByItem);

                Debug.Assert(hierarchicalGroupByControlNode != null, "CanDrop should have returned false");
                if (hierarchicalGroupByControlNode == null)
                {
                    throw new DataGridInternalException("A HierarchicalGroupByControlNode must exist for every level.");
                }

                hierarchicalGroupByControlNode.ShowFarDropMark(mousePosition);
            }
        }
        void IDropTarget.DragOver(UIElement draggedElement, Point mousePosition)
        {
            ColumnManagerCell cell = draggedElement as ColumnManagerCell;

            if (cell != null)
            {
                HierarchicalGroupByControl parentGBC = GroupingHelper.GetHierarchicalGroupByControl(this);

                if (parentGBC == null)
                {
                    throw new DataGridInternalException("The hierarchical group-by control node must be rooted by a HierarchicalGroupByControl.");
                }

                HierarchicalGroupByControlNode hierarchicalGroupByControlNode = parentGBC.GetHierarchicalGroupByControlNodeFromColumnManagerCell(cell);

                if (hierarchicalGroupByControlNode == null)
                {
                    throw new DataGridInternalException("A HierarchicalGroupByControlNode must exist for every level.");
                }

                hierarchicalGroupByControlNode.ShowFarDropMark(cell, mousePosition);
            }
            else
            {
                HierarchicalGroupByItem hierarchicalGroupByItem = draggedElement as HierarchicalGroupByItem;
                if (hierarchicalGroupByItem == null)
                {
                    return;
                }

                HierarchicalGroupByControlNode draggedHierarchicalGroupByControlNode = HierarchicalGroupByItem.GetParentHierarchicalGroupByControlNode(hierarchicalGroupByItem);

                if (draggedHierarchicalGroupByControlNode == null)
                {
                    throw new DataGridInternalException("draggedHierarchicalGroupByControlNode is null.");
                }

                if (draggedHierarchicalGroupByControlNode.GroupLevelDescriptions == this.GroupLevelDescriptions)
                {
                    this.ShowFarDropMark(mousePosition);
                }
                else
                {
                    // This HierarchicalGroupByItem does not belong this parent HierarchicalGroupByControlNode, display the DropMark on the correct one
                    HierarchicalGroupByControl parentGBC = GroupingHelper.GetHierarchicalGroupByControl(this);

                    if (parentGBC == null)
                    {
                        throw new DataGridInternalException("The hierarchical group-by control node must be rooted by a HierarchicalGroupByControl.");
                    }

                    HierarchicalGroupByControlNode hierarchicalGroupByControlNode = parentGBC.GetHierarchicalGroupByControlNodeFromHierarchicalGroupByItem(hierarchicalGroupByItem);

                    Debug.Assert(hierarchicalGroupByControlNode != null, "CanDrop should have returned false");
                    if (hierarchicalGroupByControlNode == null)
                    {
                        throw new DataGridInternalException("A HierarchicalGroupByControlNode must exist for every level.");
                    }

                    hierarchicalGroupByControlNode.ShowFarDropMark(mousePosition);
                }
            }
        }