<?xml version="1.0" encoding="utf-8" ?>
<configuration>
</configuration>

string sqlServer= System.Configuration.ConfigurationSettings.AppSettings["MySQLServer"]; 

string sqlServer = config.AppSettings.Settings["MySQLServer"].Value;

//創建對象
System.Configuration.Configuration  config = System.Configuration.ConfigurationManager.OpenExeConfiguration(System.Configuration.ConfigurationUserLevel.None);
//寫入
config.AppSettings.Settings["MySQLServer"].Value = "XXX";
//更新
config.Save(System.Configuration.ConfigurationSaveMode.Modified);
//刷新
System.Configuration.ConfigurationManager.RefreshSection("appSettings");