修改现有xml节点的值

XmlDocument doc1 = new XmlDocument();
                doc1.Load(Server.MapPath("~/etao/IncrementIndex.xml"));
                //修改商品
                XmlNodeList nodes1 = doc1.GetElementsByTagName("outer_id");
                bool ismodif = false;
                foreach (XmlNode xn in nodes1)//遍历任何子节点 
                {
                    XmlElement xe = (XmlElement)xn;//将子节点类型转换为XmlElement类型 
                    if (xe.InnerText.Equals(pid))
                    {
                        xe.SetAttribute("action", "upload");//则修改该属性为“delete”
                        XmlNodeList modified = doc1.GetElementsByTagName("modified");
                        modified.Item(0).InnerText = DateTime.Now.ToString();
                        doc1.Save(Server.MapPath("~/etao/IncrementIndex.xml"));

xml:

 <?xml version="1.0" encoding="utf-8" ?> 
- <root>
  <version>1.0</version> 
  <modified>2011-7-4 9:42:54</modified> 
  <seller_id>官网</seller_id> 
  <cat_url>etao/SellerCats.xml</cat_url> 
  <dir>/etao/item/</dir> 
- <item_ids>
  <outer_id action="upload">ID1000018452</outer_id> 
  <outer_id action="upload">ID1000018413</outer_id> 
  </item_ids>
  </root>