Code

private void fc_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
{
  // we have the screen point of the mouse, but need it in document C.S.
  Point clientPt = fc.PointToClient(new Point(e.X, e.Y));
  PointF docPt = fc.ClientToDoc(clientPt);

  // if there is a box here
  Box box = fc.GetBoxAt(docPt);
  if (box != null && e.Data.GetDataPresent(DataFormats.Text))
  {
    string s = (string)e.Data.GetData(DataFormats.Text);
    box.Text = s;
  }
}