protected void btnSave_Click(object sender, EventArgs e)
{
LopDTO tb = new LopDTO();
tb.MaLop = Convert.ToString(txtmalop.Text);
tb.TenLop = Convert.ToString(txttenlop.Text);
tb.KhoiLop = ddlkhoi.Text;
bll.SaveLop(tb);
Clear();
FillGridView();
}