XmlTextWriter保存到一个文件失败

Question

  XmlTextWriter writer = new XmlTextWriter("d:\\ftpconfig.xml", null);
       writer.Formatting = Formatting.Indented;
       //书写根元素
       writer.WriteStartElement("FTP配置");
       //开始一个元素     
       writer.WriteStartElement("FTP1");
       writer.WriteStartAttribute("remoteip"); writer.WriteString("aa");
       writer.WriteStartAttribute("user"); writer.WriteString("user");      
       writer.WriteEndAttribute();
       writer.WriteEndElement();         writer.WriteFullEndElement();
       writer.Close();