编程技巧：使用LINQ如何通过多次调用GroupBy实现分组嵌套

using System;
using System.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        public class S
        {
            public int Year;
            public int Month;
            public int Day;
        }
        
        static void Main(string[] args)
        {
            var m = new []{
                new S{Year = 2000, Month = 1, Day = 10},
                new S{Year = 2000, Month = 2, Day = 10},
                new S{Year = 2010, Month = 1, Day = 1},
                new S{Year = 2010, Month = 2, Day = 1},
                new S{Year = 2010, Month = 1, Day = 2},
                new S{Year = 2010, Month = 2, Day = 2},
                new S{Year = 2000, Month = 1, Day = 2},
                new S{Year = 2000, Month = 2, Day = 2},
            };

            var q2 =
                from s in m
                group s by s.Year into YearGroup
                select new
                {
                    Year = YearGroup.Key,
                    MonthGroups =
                        from s2 in YearGroup
                        group s2 by s2.Month into MonthGroup
                        select new
                        {
                            Month = MonthGroup.Key,
                            Days =
                                from s3 in MonthGroup
                                orderby s3.Day
                                select s3.Day
                        }
                };

            var q = m.GroupBy(
                    s => s.Year,
                    (Year, YearGroup) => new
                    {
                        Year,
                        MonthGroups =
                            YearGroup.GroupBy(
                                s2 => s2.Month,
                                (Month, MonthGroup) => new
                                {
                                    Month,
                                    Days = MonthGroup.OrderBy(s3 => s3.Day).Select(s3 => s3.Day)
                                }
                            )
                    }
                );

            foreach (var elem in q)
            //foreach (var elem in q2)
            {
                Console.WriteLine("Year = {0}", elem.Year);
                foreach (var elem2 in elem.MonthGroups)
                {
                    Console.WriteLine("\tMonth = {0}", elem2.Month);
                    foreach (var day in elem2.Days)
                        Console.WriteLine("\t\tDay = {0}", day);
                }
            }
        }
    }
}
//Year = 2000
//        Month = 1
//                Day = 2
//                Day = 10
//        Month = 2
//                Day = 2
//                Day = 10
//Year = 2010
//        Month = 1
//                Day = 1
//                Day = 2
//        Month = 2
//                Day = 1
//                Day = 2