FNC ListBox MultiSelect and Drag Drop

I'm trying to implement a drag and drop for multiple items in a list box.

In my test there are two list boxes, ListBoxLeft and ListBoxRight, populated with default data.  Interaction.DragDropMode = ldmMove, MultiSelect = True; DragMode = dmAutomatic.

The implementation is in the OnAfterDropItem event.  This is the code for the ListBoxLeft.

procedure TfrmMain.ListBoxLeftAfterDropItem(Sender: TObject; AFromItem,
  AToItem: TTMSFNCListBoxItem);
  iSelectedIndex: Integer;
  iIndex: Integer;
  iCount: Integer;
      iCount := ListBoxRight.SelectedItemCount;
      for iSelectedIndex := iCount - 1 downto 0 do
        iIndex := ListBoxRight.SelectedItems[iSelectedIndex].Index;



The code for ListBoxRight is similar, with Left and Right swapped.

It is not working as I expected.  With one item selected in a list,  I thought that going into the OnBeforeDropItem,  SelectedItemCount for the source would = 1, and in going into OnAfterDropItem, SelectedItemCount for the source would equal 0, as the item has now been deleted in the move.  Similarly if 2 items were selected, I expected the could to be 2 before the drop and 1 after the drop.

This is not what I am seeing.  SelectItemCount is sometimes changed to account for the first move, and sometimes not.

AFromItem.Index = the item index in OnBeforeDropItem, and -1 in OnAfterDropItem.

If the last item in the list is being moved
        iIndex := ListBoxRight.SelectedItems[iSelectedIndex].Index;
generates an AV.

I tried changing
      for iSelectedIndex := iCount - 1 downto 0 do
      for iSelectedIndex := iCount - 2 downto 0 do

This stopped the AVs, but now sometimes misses moving an item, depending on the value in SelectedItemCount.

I haven't been able to find a pattern to predict when the values in SelectedItemCount are what I expect and what I sometimes get.

I am missing something, or is there something not quite right in the DragDrop handling ?



You simply need to enable Interaction.DragDropMode := tdmMove to enable drag & drop between listboxes. There is no need to handle this manually.
This only allows one item to be dragged and dropped. I am trying to handle multiple items.

I've managed to work something out.