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 != 0)
                    {
                        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);
        }