void NapLop()
{
DataTable dt = new DataTable();
bus = new BUS_Diem();
dt = bus.showdata_Lop();
var item = new Dictionary <string, string>();
foreach (DataRow dr in dt.Rows)
{
item.Add(dr["MaL"].ToString(), dr["MaL"].ToString() + " - " + dr["TenL"].ToString());
}
cbMaL.DataSource = new BindingSource(item, null);
cbMaL.DisplayMember = "Value";
cbMaL.ValueMember = "Key";
}