private static IEnumerable<ScheduleMethod> onMouseLeftMethod(double value)
{
    yield return 1;
}

static void Main(string[] args)
{
    EngFun.engine_open(null, null);
    EngFun.level_load(null);

    // ....

    // register this method on the event
    EngVar.on_mouse_left += onMouseLeftMethod;

    // ....

    // unregister this method on the event
    EngVar.on_mouse_left -= onMouseLeftMethod;

    // clear all registered method on this event...
    EngVar.on_mouse_left.Events.Clear();
}