C# と VB.NET の質問掲示板

using System;
using System.Linq;

普通にDateでgroupbyした結果を、ゆっくり料理すればいいような気がします。

namespace ConsoleApplication3
{
    class Record
    {
        public string Date { get; set; }
        public int Number { get; set; }
        public int Value { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var data = new[]
            {
                new Record { Date = "1/1", Number = 1, Value = 234 },
                new Record { Date = "1/1", Number = 2, Value = 12 },
                new Record { Date = "1/1", Number = 3, Value = 455 },
                new Record { Date = "1/2", Number = 1, Value = 235 },
                new Record { Date = "1/2", Number = 2, Value = 13 },
                new Record { Date = "1/2", Number = 3, Value = 453 },
                new Record { Date = "1/3", Number = 1, Value = 229 },
                new Record { Date = "1/3", Number = 2, Value = 15 },
                new Record { Date = "1/3", Number = 3, Value = 444 },
            };

            var results = from record in data
                          orderby record.Number
                          group record by record.Date;
            foreach (var result in results)
            {
                Console.WriteLine(result.Key);
                foreach (var g in result)
                {
                    Console.WriteLine("\t{0}, {1}", g.Number, g.Value);
                }
            }

        }
    }
}