private void DragSourceManager_DroppedOutside(object sender, EventArgs e)
        {
            HierarchicalGroupByControlNode hierarchicalGroupByControlNode = HierarchicalGroupByItem.GetParentHierarchicalGroupByControlNode(this);

            if (hierarchicalGroupByControlNode == null)
            {
                throw new DataGridInternalException();
            }

            bool allowGroupingModification = hierarchicalGroupByControlNode.IsGroupingModificationAllowed;

            if (!allowGroupingModification)
            {
                return;
            }

            ObservableCollection <GroupDescription> groupDescriptions = this.ParentGroupDescriptions;

            if (groupDescriptions != null)
            {
                // Get the HierarchicalGroupByControl before removing us from it
                HierarchicalGroupByControl parentGBC = GroupingHelper.GetHierarchicalGroupByControl(this);

                GroupLevelDescription groupLevelDescription = this.Content as GroupLevelDescription;

                DataGridContext dataGridContext = DataGridControl.GetDataGridContext(this);

                DataGridControl parentDataGridControl = (dataGridContext != null)
          ? dataGridContext.DataGridControl
          : null;

                GroupingHelper.RemoveGroupDescription(groupDescriptions, groupLevelDescription, parentDataGridControl);

                // Notify groups have changed for NoGroupContent
                parentGBC.UpdateHasGroups();

                // Update the HasGroups property
                Debug.Assert(parentGBC != null);
                if (parentGBC == null)
                {
                    throw new DataGridInternalException("The hierarchical group-by item must be rooted by a HierarchicalGroupByControl.");
                }
            }
        }
        void IDropTarget.Drop(UIElement draggedElement)
        {
            ColumnManagerCell cell = draggedElement as ColumnManagerCell;

            if (cell == null)
            {
                return;
            }

            HierarchicalGroupByControl parentGBC = GroupingHelper.GetHierarchicalGroupByControl(this);

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

            DataGridContext dataGridContext = DataGridControl.GetDataGridContext(this);

            DataGridControl parentGrid = (dataGridContext != null)
        ? dataGridContext.DataGridControl
        : null;

            GroupingHelper.AppendNewGroupFromColumnManagerCell(cell, parentGrid);

            // Notify groups have changed for NoGroupContent
            parentGBC.UpdateHasGroups();

            HierarchicalGroupByControlNode hierarchicalGroupByControlNode = parentGBC.GetHierarchicalGroupByControlNodeFromColumnManagerCell(cell);

            if (hierarchicalGroupByControlNode == null)
            {
                return;
            }

            hierarchicalGroupByControlNode.HideFarDropMark(cell);

            this.HideFarDropMark();
        }
        void IDropTarget.Drop(UIElement draggedElement)
        {
            ColumnManagerCell draggedColumnManagerCell = draggedElement as ColumnManagerCell;

            if (m_dropMarkAdorner != null)
            {
                GroupLevelDescription draggedOverGroupLevelDescription = this.Content as GroupLevelDescription;

                DropMarkAlignment alignment = m_dropMarkAdorner.Alignment;
                this.HideDropMark();

                if (draggedColumnManagerCell != null)
                {
                    DataGridContext dataGridContext = DataGridControl.GetDataGridContext(this);

                    DataGridControl parentGrid = (dataGridContext != null)
            ? dataGridContext.DataGridControl
            : null;

                    GroupingHelper.AddNewGroupFromColumnManagerCell(draggedColumnManagerCell, draggedOverGroupLevelDescription, alignment, parentGrid);
                }
                else
                {
                    HierarchicalGroupByItem draggedGroupByItem = draggedElement as HierarchicalGroupByItem;

                    if (draggedGroupByItem == null)
                    {
                        return;
                    }

                    GroupLevelDescription draggedGroupLevelDescription = draggedGroupByItem.Content as GroupLevelDescription;

                    DataGridContext dataGridContext = DataGridControl.GetDataGridContext(this);

                    DataGridControl parentDataGridControl = (dataGridContext != null)
            ? dataGridContext.DataGridControl
            : null;

                    GroupLevelDescription destinationGroupLevelDescription = this.Content as GroupLevelDescription;

                    GroupingHelper.MoveGroupDescription(this.ParentColumns, this.ParentGroupDescriptions,
                                                        destinationGroupLevelDescription, alignment,
                                                        draggedGroupLevelDescription, parentDataGridControl);
                }
            }
            else
            {
                // We try to add a new Group which is not in the current GroupLevelDescriptions
                if (draggedColumnManagerCell == null)
                {
                    return;
                }

                DataGridContext dataGridContext = DataGridControl.GetDataGridContext(this);

                DataGridControl parentGrid = (dataGridContext != null)
          ? dataGridContext.DataGridControl
          : null;

                GroupingHelper.AppendNewGroupFromColumnManagerCell(draggedColumnManagerCell, parentGrid);
            }

            HierarchicalGroupByControl parentGBC = GroupingHelper.GetHierarchicalGroupByControl(this);

            Debug.Assert(parentGBC != null);
            if (parentGBC == null)
            {
                throw new DataGridInternalException("A hierarchical group-by item must be rooted by a HierarchicalGroupByControl.");
            }

            // Notify groups have changed for NoGroupContent
            parentGBC.UpdateHasGroups();
        }