public class Stage {
    ...
    public Control Control(int num, DataBase dataBase) {
        IList<Control> res = dataBase.db.Query<Control>(
                delegate(Control control) {
                    return control.Number == num && control.Stage == this;
                }
            );
        if (res.Count == 0)
            throw new DoesntExist();
        else return res[0];
    }
    ...
}