C#

public class DragDropTreeView : TreeView
{
  public DragDropTreeView()
  {
    AllowDrop = true;
  }
  // ...
  protected override void OnItemDrag(ItemDragEventArgs e)
  {
    TreeNode node = e.Item as TreeNode;
    if (node != null)
    {
      DataObject data = new DataObject("TreeNode", node);
      DoDragDrop(data, DragDropEffects.Copy);
    }
  }
  protected override void OnDragOver(DragEventArgs e)
  {
    if (e.Data.GetDataPresent("TreeNode") &&
       (e.AllowedEffect & DragDropEffects.Copy) != 0)
      e.Data.Effect = DragDropEffects.Copy;
  }
  protected override void OnDragDrop(DragEventArgs e)
  {
    TreeNode node = e.Data.GetData("TreeNode") as TreeNode;
    if (node != null)
    {
      Point p = PointToClient(new Point(e.X, e.Y));
      TreeNode parent = GetNodeAt(p);
      if (parent != null) parent.Nodes.Add(node);
    }
  }
}

email = null;
if (email)
{
  // ...
}