var fileMap = new ExeConfigurationFileMap(filename)

 static void Main(string[] args)
        {
            AppDomainSetup setup = new AppDomainSetup();
            setup.ConfigurationFile = "App1.config";
            AppDomain newDomain = AppDomain.CreateDomain("newDomain", null, setup);
            newDomain.DoCallBack(
                new CrossAppDomainDelegate(() => { Console.WriteLine("Config: {0}", AppDomain.CurrentDomain.SetupInformation.ConfigurationFile); })
            );
        }