关于.net中XML遍历的问题

public   void showXML(string path)
            {
                XmlDocument xd = new XmlDocument();
                xd.Load(path);
                XmlNodeList xnl = xd.DocumentElement.ChildNodes;                foreach (XmlNode xn in xnl)
                {
                   
                    Response.Write(xn.Attributes["name"].Value.ToString()+"<br>");
                    XmlNode child = xn.FirstChild;
                    
                    NodeOperate(child);
                               }
            }    public  void NodeOperate(XmlNode xn1)
    {        if (xn1.HasChildNodes == true)
        {
            Response.Write(xn1.Name + "<br>");
            Response.Write("<br>");
            XmlNode childNode = xn1.FirstChild;            NodeOperate(childNode);        }
        else
        {
            Response.Write(xn1.Name + "<br>");
            Response.Write(xn1.InnerText);
            Response.Write("<br>");
            if (xn1.NextSibling != null)
            {                NodeOperate(xn1.NextSibling);
            }
            else
            {
                while (xn1.NextSibling == null) //遍历到底后移到其父节点的下一个元素
                {
                    xn1 = xn1.ParentNode;
                }
                NodeOperate(xn1.NextSibling);
            }        }
    }XML文件：
<root>
  <person name="张三">
    <password>12</password>
    <email>[email protected]</email>
    <detail>
      <hobby>吃</hobby>
      <other>无</other>
    </detail>
  </person>
  <person name="李四">
    <password>1212</password>
    <email>[email protected]</email>
    <detail>
      <hobby>电脑</hobby>
      <other>无</other>
    </detail>
  </person>
</root>问题是当我运行的时候程序报错，说这句while (xn1.NextSibling == null)未将对象引用设置到对象的实例，不知道怎么解决？