App.config中：
<appSettings>
<add key="connectionstring" value="Data Source=127.0.0.1;Initial Catalog=DatabaseName;Persist Security Info=True;
User ID=sa;Password=sa"/>
</appSettings>
调用的地方：
private string connStr = ConfigurationManager.AppSettings["connectionstring"];
程序集引用：
using System.Configuration;
原文：https://www.cnblogs.com/tiancaige/p/12719324.html