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