c# 更新web.config

 /// <summary>
         /// 添加和更新配置文件web.config的appSettings，缺点是会删除注释
         /// </summary>
         /// <param name="key"></param>
         /// <param name="value"></param>
         public static void AddUpdateAppSettings(string key, string value)
         {
             try
             {
                 var configFile = WebConfigurationManager.OpenWebConfiguration("~");
                 var settings = configFile.AppSettings.Settings;
                 try
                 {
                     if (settings[key] == null)
                     {
                         settings.Add(key, value);
                     }
                     else
                     {
                         settings[key].Value = value;
                     }
                     configFile.Save(ConfigurationSaveMode.Modified);
                     ConfigurationManager.RefreshSection(configFile.AppSettings.SectionInformation.Name);
                 }
                 catch (Exception ex)
                 {
                     if (settings.Count != )
                     {
                         NLog.Error("配置文件写入失败,需要写入文件的配置信息：===============================");
                         foreach (string tkey in settings.AllKeys)
                         {
                             string tvalue = settings[tkey].Value;
                             NLog.Error(string.Format("Key: {0} Value: {1}", tkey, tvalue));
                         }
                     }
                     throw;
                 }
             }
             catch (Exception ex)
             {
                 NLog.Error("具体失败内容:" + ex);
             }
         }

         /// <summary>
         /// 添加和更新web.config的appSettings，且不删除注释
         /// </summary>
         /// <param name="key"></param>
         /// <param name="value"></param>
         public static void UpdateAppSetting(string key, string value)
         {
             try
             {
                 Configuration config = WebConfigurationManager.OpenWebConfiguration("~");
                 SaveUsingXDocument(key, value, config.AppSettings.ElementInformation.Source);
             }
             catch (Exception ex)
             {
                 NLog.Error("更新配置失败" + ex);
             }
         }
         private static void SaveUsingXDocument(string key, string value, string fileName)
         {
             XDocument document = XDocument.Load(fileName);
             if (document.Root == null)
             {
                 return;
             }
             var appSettings = document.Root.Element("appSettings");
             if (appSettings == null) return;
             XElement appSetting = appSettings.Elements("add").FirstOrDefault(x => x.Attribute("key").Value == key);
             if (appSetting != null)
             {
                 appSetting.Attribute("value").Value = value;
             }
             else
             {
                 XElement ele = new XElement("add", new XAttribute("key", key), new XAttribute("value", value));
                 appSettings.Add(ele);
             }
             document.Save(fileName);
         }