using System;

using System.Xml;

using System.Xml.Serialization;

using System.IO;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

using System.Xml.Linq;

using System.Data;

​

public class Program

{

    public static void Main()

    {

        var fooList = new List<Foo> {

            new Foo { SomeDate = DateTime.Now.AddMonths(0), SomeValue = 1 },

            new Foo { SomeDate = DateTime.Now.AddMonths(0), SomeValue = 2 },

            new Foo { SomeDate = DateTime.Now.AddMonths(6), SomeValue = 3 },

            new Foo { SomeDate = DateTime.Now.AddMonths(6), SomeValue = 4 },

            new Foo { SomeDate = DateTime.Now.AddMonths(12), SomeValue = 5 },

            new Foo { SomeDate = DateTime.Now.AddMonths(12), SomeValue = 6 },

            new Foo { SomeDate = DateTime.Now.AddMonths(14), SomeValue = 7 },

            new Foo { SomeDate = DateTime.Now.AddMonths(14), SomeValue = 8 }

        };

​

        var results = fooList.GroupBy(g=> new { g.SomeDate.Year })

            .Select(s=> 

                    new 

                    {

                        Year = s.Key, 

                        TheContent = s.GroupBy(x=>x.SomeDate.Month)

                                      .Select(m=> 

                                              new 

                                              { 

                                                  Month =  m.Key, 

                                                  TheSum = m.Sum(e=>e.SomeValue)

                                              }).ToList()

                    });

        foreach(var result in results)

        {

            foreach(var content in result.TheContent)

            {

                Console.WriteLine("{0} - {1}", result.Year, content);

            }

        }

    }   

}

​

​

​

​

public class Foo

{

    public DateTime SomeDate { get; set; }

    public int SomeValue { get; set; }

}

​

​