using System;
using System.Linq;
using System.Collections.Generic;
 
public class Test
{
	public static void Main()
	{
        var oldList = new List<int?> { 2, 3, 4, 5 };
        var newList = new List<int?> { 1, 2 };
        var result = from all in oldList.Union(newList).OrderBy(num => num)
                     join o in oldList on all equals o into gjOld
                     from oldOuter in gjOld.DefaultIfEmpty((int?)null)
                     join n in newList on all equals n into gjNew
                     from newOuter in gjNew.DefaultIfEmpty((int?)null)
                     select new { newVal = newOuter, oldVal = oldOuter };
         foreach(var x in result)
             Console.WriteLine("newVal: {0} oldVal: {1}", x.newVal, x.oldVal);
	}
}

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