using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

using MoreLinq;

​

public class Program

{

    public static void Main()

    {

        var oldCol = new List<MyCollection> {

            new MyCollection{ id = 1, name = "firstOld" },

            new MyCollection{ id = 2, name = "secondOld" },

            new MyCollection{ id = 3, name = "thirdOld" },

        };

        var newCol = new List<MyCollection> {

            new MyCollection{ id = 1, name = "firstNew" },

            new MyCollection{ id = 3, name = "thirdNew" },

            new MyCollection{ id = 4, name = "fourthNew" },

            new MyCollection{ id = 5, name = "fifthNew" },

        };

        var results = oldCol.ZipLongest(newCol, (oldItem, newItem) => new Tuple<MyCollection, MyCollection>(oldItem, newItem) /*oldItem != null && newItem != null && oldItem.id == newItem.id*/).ToList();

        var jsonString = JsonConvert.SerializeObject(

           results, Formatting.Indented,

           new JsonConverter[]{});

        Console.Write(jsonString.ToString());

    }

    class MyCollection {

        public int id  {get; set;}

        public string name {get;set;}       

    }

}