using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    class Record

    {

        public int id {get; set;}

        public string name {get; set;}

        public DateTime date {get; set;}

    }

    public static void Main()

    {

        var records = new List<Record>()

        {

            new Record()

            {

                id = 1,

                name = "abc",

                date = new DateTime(2020, 8, 8)

            },

            new Record()

            {

                id = 2,

                name = "def",

                date = new DateTime(2020, 8, 9)

            },

            new Record()

            {

                id = 1,

                name = "def",

                date = new DateTime(2020, 8, 9)

            }

        };

        var nested = records.GroupBy(r => r.id).Select(g =>

            new

            {

               id = g.Key,

               children = g.Select(cr =>

               new 

               {

                 name = cr.name,

                 date = cr.date

               }).ToList()

            }).ToList();

        foreach (var parent in nested)

        {

            Console.WriteLine($"Parent #{parent.id}");

​

            Console.WriteLine("Children");

            foreach (var child in parent.children)

            {

                Console.WriteLine("Child");

                Console.WriteLine(child.name);

                Console.WriteLine(child.date);

            }

        }

    }

}