读取：
var imgSizeStr = ConfigurationManager.AppSettings["UploadImgSize"];
写入：
Configuration config = WebConfigurationManager.OpenWebConfiguration("/");
AppSettingsSection app = config.AppSettings;
app.Settings["UploadImgSize"].Value = value.ToString();
config.Save(ConfigurationSaveMode.Modified);
原文：https://www.cnblogs.com/come-on-come-on/p/10216139.html