在web上 需要用Resources来进行加载，否则读取不到

  /// <summary>
        /// 加载xml文档
        /// </summary>
        /// <returns></returns>
        public static XmlDocument ReadAndLoadXml()
        {
            
            XmlDocument doc = new XmlDocument();
            TextAsset textAsset = (TextAsset)Resources.Load("XMLFile1");    
            Debug.Log("加载xml文档");
            doc.LoadXml(textAsset.text); 
            return doc;

        }