Example: Label

public class Application : Gtk.Window {

public Application () {
// Prepare Gtk.Window:
this.title = "My Gtk.Label";
this.window_position = Gtk.WindowPosition.CENTER;
this.destroy.connect (Gtk.main_quit);
this.set_default_size (350, 70);

// The Label:
Gtk.Label label = new Gtk.Label ("<b>Hello, world!</b>");
label.set_use_markup (true);
label.set_line_wrap (true);
this.add (label);
}

public static int main (string[] args) {
Gtk.init (ref args);

Application app = new Application ();
app.show_all ();
Gtk.main ();
return 0;
}
}

 valac --pkg gtk+-3.0 Gtk.Label.vala

See: