using Gtk; [GtkTemplate (ui="/org/test/appwindow.ui")] class TestWindow : Gtk.ApplicationWindow { public TestWindow(Gtk.Application app) { Object(application: app); } } class TestApp : Gtk.Application { public override void activate() { var window = new TestWindow(this); window.show_all(); } } int main(string[] args) { var app = new TestApp(); return app.run(); }