using System;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

public class Program

{

    public class C

    {

        public int Id { get; set; }

        public string Title { get; set; }

    }

    public static void Main()

    {

        var list = new List<C>()

        {

            new C { Id = 1, Title = "11" },

            new C { Id = 1, Title = "12" },

            new C { Id = 1, Title = "13" },

            new C { Id = 2, Title = "21" },

            new C { Id = 2, Title = "22" },

            new C { Id = 2, Title = "23" },

        };

        var group = list.ToLookup(x => x.Id, x => x.Title);

        Console.WriteLine(JsonConvert.SerializeObject(group));

        Console.WriteLine("Hello World");

    }

}