private   string   strconn="Provider=SQLOLEDB.1;User   ID=sa;password=;Initial   Catalog=通信录;Data   Source=lhz";   
  public   OleDbConnection   cn   =   new   OleDbConnection();               
  public   OleDbCommand   cm   =   new   OleDbCommand()   ;   
  private   OleDbDataAdapter   da   =   new   OleDbDataAdapter();   
  Xml     XMLFile1=new   Xml();   
  cn.ConnectionString=strconn;   
  cm.Connection=cn;   
  cm.CommandText="select   CustID,CustName,QQ,Email   from   tongxin";   
  da.SelectCommand=cm;   
  DataSet   ds=new   DataSet();   
  da.Fill(ds,"tongxin");   
  try   
  {   
  ds.WriteXml(Request.MapPath(XMLFile1.xml));   
  }   
  catch(Exception)   
  {   
  }   
  finally   
  {   
  DataGrid1.DataSource=ds;   
  DataGrid1.DataBind();   
  }

------解决方案--------------------
将数据读取到dataset里
然后循环插入XML

------解决方案--------------------
xmltextwriter,可以writestartdoc,writesttartelement,writevalue,
先写跟节点，然后写子节点，然后关闭写子节点，关闭写根节点

------解决方案--------------------
 //声明一个XmlDocument空对象
       protected XmlDocument XmlDoc = new XmlDocument();

       /// <summary>
       /// 构造函数，导入Xml文件
       /// </summary>
       /// <param name="xmlFile">文件虚拟路径</param>
       public XmlOp(string xmlFile)
       {
           try
           {
               XmlDoc.Load(GetXmlFilePath(xmlFile));   //载入Xml文档
           }
           catch (System.Exception ex)
           {
               throw ex;
           }
       }
       /// <summary>
       /// 析构函数
       /// </summary>
       ~XmlOp()
       {
           XmlDoc = null;  //释放XmlDocument对象
       }

       /// <summary>
       /// 保存文件
       /// </summary>
       /// <param name="filePath">文件虚拟路径</param>
       public void Save(string filePath)
       {
           try
           {
               XmlDoc.Save(GetXmlFilePath(filePath));
           }
           catch (System.Exception ex)
           {
               throw ex;
           }
       }

       /// <summary>
       /// 返回Xml文件实际路径
       /// </summary>
       /// <param name="xmlFile">文件虚拟路径</param>
       /// <returns></returns>
       public string GetXmlFilePath(string xmlFile)
       {
           return HttpContext.Current.Server.MapPath(xmlFile);
       }

       /// <summary>
       /// 根据Xml文件的节点路径，返回一个DataSet数据集
       /// </summary>
       /// <param name="XmlPathNode">Xml文件的某个节点</param>
       /// <returns></returns>
       public DataSet GetDs(string XmlPathNode)
       {
           DataSet ds = new DataSet();
           try
           {