public class SqlDataProduct
    {
        public string name { get; set; }
        public string amount { get; set; }
        public List<string> listGroup { get; set; }
    }

       using (SqlConnection conn = new SqlConnection(m_connectionString))
                {
                    conn.Open();
                    using (SqlCommand queryProductResponse = new SqlCommand(queryProduct, conn))

                    {
                        SqlDataReader reader = queryProductResponse.ExecuteReader();

                        if (reader.HasRows)
                        {
                            while (reader.Read())
                            {
                                int adressId = 0;

                                SqlDataProduct sqlData = new SqlDataProduct();


                                sqlData.name = reader["Nazwa"].ToString();
                                sqlData.amount= reader["Amount"].ToString();

                                sqlData.listGroup = new List<string>();

                                using (SqlCommand queryGroupResponse = new SqlCommand(queryGroup, conn))
                                {
                                    SqlDataReader readerGroup = queryGroupResponse.ExecuteReader();
                                    if (readerGroup.HasRows)
                                    {//pobieranie struktury  kategori i ich id dla produktu
                                        while (readerGroup.Read())
                                        {

                                            sqlData.listGroup.Add(readerGroup["GN"].ToString());


                                            using (SqlCommand queryGroupNameResponse = new SqlCommand(queryGroupName, conn))
                                            {
                                                SqlDataReader readerGroupName = queryGroupNameResponse.ExecuteReader();
                                                if (readerGroupName.HasRows)
                                                {
                                                    sqlData.listGroup.Add(readerGroupName["TwG_Nazwa"].ToString());
                                                }

                                                else
                                                {
                                                    break;
                                                }
                                            }
                                        }
                                    }
                                }