/// summary>
          /// 写入web.config
          /// /summary>
          /// param name="item">appSettings等/param>
          /// param name="key">键/param>
          /// param name="value">值/param>
          public void WriteConfig(string item, string key, string value)
          {
              if (item == "")
             {
                 item = "appSettings";
             }
             Configuration config = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration(System.Web.HttpContext.Current.Request.ApplicationPath);
             AppSettingsSection appSection = (AppSettingsSection)config.GetSection(item);
             if (appSection.Settings[key] == null)
             {
                 appSection.Settings.Add(key, value);
                 config.Save();
             }
             else
             {
                 appSection.Settings.Remove(key);
                 appSection.Settings.Add(key, value);
                 config.Save();
            }
         }