public int Compare(Product first, Product second) {
            return PartialComparer.RefernceCompare(first, second) ?? PartialComparer.Comare(first.Popularity, second.Popularity) ?? PartialComparer.Comare(first.Price, second.Price) ?? PartialComparer.Comare(first.Name, second.Name) ?? 0;
        }
    }

    public static class PartialComparer {
        public static int? Comare<T>(T first, T second) {
            return Compare(Comparer<T>.Default, first, second);
        }
        public static int? Compare<T>(IComparer<T> comparer, T first, T second) {
            int ret = comparer.Compare(first, second);
            return ret == 0 ? new int?() : ret;
        }
        public static int? RefernceCompare<T>(T first, T second) where T : class {
            return first == second ? 0 : (first == null ? -1 : (second == null ? 1 : new int?()));
        }
    }