C# 获取XML节点的属性值

XmlDocument doc = new XmlDocument();
                doc.Load(HttpContext.Current.Server.MapPath("~/App_Data/" + xmlName + ".xml"));
                XmlNodeList nodeList = doc.SelectNodes(xpath);
                foreach (XmlNode sub in nodeList)
                {
                    if (value.IndexOf(sub.Attributes["value"].Value) != -1)
                    {
                        value = sub.Attributes["value"].Value;
                    }
                }