What is wrong with drag and drop

private void dgv_Functions_MouseDown(object sender, MouseEventArgs e)
      {
          dgv_Functions.DoDragDrop(dgv_Functions.SelectedRows,
          DragDropEffects.Copy);
      }

private void txt_Query_DragEnter(object sender, DragEventArgs e)
      {
         e.Effect = DragDropEffects.Copy;
      }

private void txt_Query_DragDrop(object sender, DragEventArgs e)
      {
          if (e.Data.GetDataPresent(typeof(System.String)))
           {
              txt_Query.Text =
                  (System.String)e.Data.GetData(typeof(System.String));
           }
      }