创建XML文件没有输出内容


  public void get_datasource(int proid, int take, int skip,string xmlpath)
        {
            //实例化数据访问类
            Box_Case_JoinCode db = new Box_Case_JoinCode();
            //创建xml对象
            XmlDocument doc = new XmlDocument();
            XmlWriterSettings setting = new XmlWriterSettings();
            setting.Indent = true;
            XmlWriter xw;
            try{
                using (xw = XmlWriter.Create(xmlpath, setting))
                {
                    //创建第一层节点
                    XmlElement element = doc.CreateElement("ProductList");//创建ProductList节点
                    element.SetAttribute("version", "3.0");
                    //得到产品信息
                    DefWl_bs_productinfo info = db.get_product_info(proid);
                    //------2.读取第二节点的产品信息：产品基本信息
                    XmlElement product_element = doc.CreateElement("Product");
                    product_element.SetAttribute("Id", info.Id.ToString());
                    product_element.SetAttribute("Productname", info.Productname);
                    product_element.SetAttribute("Typename", info.Typename);
                    //添加给第一节点
                    element.AppendChild(product_element);
                    //得到根据条件查询到的casecode的datatable
                    DataTable casecode_dt = db.get_casecode(proid, take, skip);
                    for (int i = 0; i < casecode_dt.Rows.Count; i++)
                    {
                        DataRow row = casecode_dt.Rows[i];
                        //------3.读取第三节点的casecode箱码信息
                        XmlElement casecode_element = doc.CreateElement("CaseCode");
                        casecode_element.SetAttribute("casecode", row[0].ToString());
                        product_element.AppendChild(casecode_element);//添加
                        //根据条件查询到casecode对应的boxFWCode
                        DataTable box_table = db.get_boxCodeDataSource(proid, take, skip, row[0].ToString());
                        foreach (DataRow box_row in box_table.Rows)
                        {
                            XmlElement box_col = doc.CreateElement("Box_Code");
                            box_col.SetAttribute("boxFWCode", box_row[0].ToString());
                            box_col.SetAttribute("ProductName", box_row[1].ToString());
                            casecode_element.AppendChild(box_col);                            doc.WriteTo(xw);                        }
                    }
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }        }
内有异常，没有错误，正常运行，却没有输出内容，头一回弄这东西，不是太明白。求解……