using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<Product> products = new List<Product>

           {

               new Product { ProductID = 1, CategoryID = 10, CategoryName = "Electronics", DateLastModified = new DateTime(2014,06,12) },

               new Product { ProductID = 2, CategoryID = 11, CategoryName = "Beverages", DateLastModified = new DateTime(2014,05,01) },

               new Product { ProductID = 3, CategoryID = 10, CategoryName = "Electronics", DateLastModified = new DateTime(2014,06,28) },

               new Product { ProductID = 4, CategoryID = 11, CategoryName = "Beverages", DateLastModified = new DateTime(2014,06,12) },

               new Product { ProductID = 5, CategoryID = 10, CategoryName = "Electronics", DateLastModified = new DateTime(2014,10,13) },

               new Product { ProductID = 6, CategoryID = 11, CategoryName = "Beverages", DateLastModified = new DateTime(2014,05,10) },

           };

​

           var query = from pro in products

                       group pro by pro.CategoryName into newGroup1

                       from newGroup2 in

                           (from prod in newGroup1

                            group prod by new { prod.DateLastModified.Year, prod.DateLastModified.Month } 

                           )

                       group newGroup2 by newGroup1.Key;

​

​

           foreach (var categories in query)

           {

               Console.WriteLine("Category Name : {0}",categories.Key);

               foreach (var modifiedDates in categories)

               {

                   Console.WriteLine("Year : {0}, Month : {1}", modifiedDates.Key.Year, modifiedDates.Key.Month);

                   foreach (var prods in modifiedDates)

                   {

                       Console.WriteLine(prods.ProductID);

                   }

               }

               Console.WriteLine();

           }

    }

}

​

public class Product

    {

        public int ProductID { get; set; }

        public int CategoryID { get; set; }

        public string CategoryName { get; set; }

        public DateTime DateLastModified { get; set; }

    }