调用线程必须为 STA，因为许多 UI 组件都需要

class Mainclass
    {
        [STAThread]
        static void Main()
        {
            Application app = new Application();
            MainWindow win = new MainWindow();

            app.Startup += new StartupEventHandler(app_Startup);
            app.Run();
        }

        static void app_Startup(object sender,StartupEventArgs e)
        {
            MessageBox.Show("fdf");
        }
    }