private void OnItemDrop(object sender, ItemDropArgs args)
{
if (args.DropTarget == null)
{
return;
}
TreeView.ItemDropStdHandler <DataItem>(args,
(item) => item.Parent,
(item, parent) => item.Parent = parent,
(item, parent) => ChildrenOf(parent).IndexOf(item),
(item, parent) => ChildrenOf(parent).Remove(item),
(item, parent, i) => ChildrenOf(parent).Insert(i, item));
}