get app.config data with configurationmanager.appsettings

右邊的選單的加入參考System.Configuration
不是using System.Configuration;

在程式內才能使用，不會報錯

configurationmanager.appsettings

string DNS = ConfigurationSettings.AppSettings.Get("DNS");

string DNS = ConfigurationManager.AppSettings["DNS"];

string DNS = Config.DNS;