using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

public class Program

{

    public class TstOb

    {

        public string Name {get;set;}

        public Guid Id {get;set;}

        public TstOb(Guid? id = null, string name = null)

        {

            this.Name = name ?? "new guy";

            this.Id = id ?? Guid.NewGuid();

        }

    }

    public static void Main()

    {

        Console.WriteLine("Hello World\n");

        var obList1 = new List<TstOb>();

        var obList2 = new List<TstOb>();

        obList1.Add(new TstOb(Guid.Empty));

        obList2.Add(new TstOb(Guid.Empty, "Mr. Dude"));

        obList1.Add(new TstOb());

        obList2.Add(new TstOb());

        obList2.Add(new TstOb());

        var finalList = obList1;

        //finalList = (from ob1 in obList1 join ob2 in obList2 on ob1.Id equals ob2.Id update ).ToList();

        //var totalList = from ob1 in obList1 join ob2 in obList2 on ob1.Id equals ob2.Id 

        var totalList = obList1.Union(obList2);

​

        // option 1: do it all in one statement

​

        finalList = (from v in obList2.Union(obList1)

                        group v by v.Id into grp

                        select grp.FirstOrDefault()).ToList();

        // option 2: do it all in two statements    

        var almostfinalList = obList1.Where(o => !(obList2.Any(oo => oo.Id == o.Id))).ToList();

        var finalList2 = obList2.Union(almostfinalList).ToList();

        Console.WriteLine("obList1 contents:\n");

        foreach(var ob in obList1){

            Console.WriteLine("name: '"+ob.Name+"'   || id: '"+ob.Id+"'\n");

        }

        Console.WriteLine("obList2 contents:\n");

        foreach(var ob in obList2){

            Console.WriteLine("name: '"+ob.Name+"'   || id: '"+ob.Id+"'\n");

        }

        Console.WriteLine("totalList contents:\n");

        foreach(var ob in totalList){

            Console.WriteLine("name: '"+ob.Name+"'   || id: '"+ob.Id+"'\n");

        }

        Console.WriteLine("finalList contents:\n");

        foreach(var ob in finalList){

            Console.WriteLine("name: '"+ob.Name+"'   || id: '"+ob.Id+"'\n");

        }

        Console.WriteLine("finalList v2 contents:\n");

        foreach(var ob in finalList2){

            Console.WriteLine("name: '"+ob.Name+"'   || id: '"+ob.Id+"'\n");

        }

    }

}