常用：https://zhuanlan.zhihu.com/p/141469940

IEnumerable<int> s = new List<int>() { 1,2,3,4,5,6,7,9};
        IEnumerable<int> res = s.Where(t => t > 5);
        int count = s.Count(t => t > 5);
        int sum = s.Sum();
        IEnumerable<int> res1 = from val in s
                                where val <= 5
                                select val;
        //Group
        var list = s.GroupBy(p => p>5).ToList();
        //print(list[0].ToList()[0]);
        //print(list[1].ToList()[0]);
        //
        List<int> res2 = s.OrderByDescending(p => p).ToList();