private bool DangNhap(string tenDangNhap, string matKhau)
 {
     dataTable = new DataTable();
     dataTable = bd.KiemTraDangNhap(ref err, tenDangNhap, matKhau);
     if (dataTable.Rows.Count > 0)
     {
         if (dataTable.Rows[0]["code"].ToString().Equals("1"))
         {
             Cls_Main.tenNhanVien = dataTable.Rows[0]["TenNhanVien"].ToString();
             Cls_Main.maTaiKhoan  = dataTable.Rows[0]["MaTaiKhoan"].ToString();
             Cls_Main.maNhanVien  = dataTable.Rows[0]["MaNhanVien"].ToString();
             return(true);
         }
     }
     return(false);
 }