C#怎么把显示子节文本值叠加

    public static  string  ShowXml()
    {
        xmlDoc = new XmlDocument();
        xmlDoc.Load("F:\\6.4迭代一\\Virtual Exchange\\TradeCenterUI\\MailSendOut.xml"); //加载xml文件
        XmlNode xn = xmlDoc.SelectSingleNode("bookshop");

        XmlNodeList xnl = xn.ChildNodes;

        foreach (XmlNode xnf in xnl)
        {
            XmlElement xe = (XmlElement)xnf;
            Console.WriteLine(xe.GetAttribute("genre"));//显示属性值 
            Console.WriteLine(xe.GetAttribute("ISBN"));

            XmlNodeList xnf1 = xe.ChildNodes;
            foreach (XmlNode xn2 in xnf1)
            {
                Console.WriteLine(xn2.InnerText);//显示子节点点文本  
            }

        }
        return xnl.ToString();

       　
    }