private void dataGridView2_CellMouseDoubleClick(object sender, DataGridViewCellMouseEventArgs e)
 {
     if (true)
     {
         string    cid = dataGridView2.Rows[e.RowIndex].Cells[0].Value.ToString();
         CourseBLL bll = new CourseBLL();
         bool      b   = bll.DeleteStuCourse(sid, cid);
         if (b)
         {
             MessageBox.Show("退课成功");
             LoadMyCourse();
         }
         else
         {
             MessageBox.Show("退课失败");
         }
     }
 }