private void dataGridView1_CellMouseDoubleClick(object sender, DataGridViewCellMouseEventArgs e)
 {
     if (true)
     {
         string    cid = dataGridView1.Rows[e.RowIndex].Cells[0].Value.ToString();
         CourseBLL bll = new CourseBLL();
         bool      b   = bll.AddStuCourse(sid, cid);
         if (b)
         {
             MessageBox.Show("选课成功");
             LoadMyCourse();
         }
         else
         {
             MessageBox.Show("选课失败");
         }
     }
 }