private void cboTimTonGiao_Click(object sender, EventArgs e)
{
cboTimTonGiao.SelectionStart = cboTimTonGiao.Text.Length;
BUS_tblHocSinh busHs = new BUS_tblHocSinh();
DataTable tb = busHs.getField("TonGiao");
cboTimTonGiao.Items.Clear();
cboTimTonGiao.Items.Add("Tất cả");
for (int i = 0; i < tb.Rows.Count; i++)
{
cboTimTonGiao.Items.Add(tb.Rows[i]["TonGiao"].ToString());
}
}