using System;
using System.Collections.Generic;
using System.Linq;

namespace TestApp
{
    class Program
    {
        static void Main(string[] args)
        {
            var list1 = new List<ItemOne>
                            {
                                new ItemOne {IDItem = 1, OneProperty = "1"},
                                new ItemOne {IDItem = 2, OneProperty = null},
                                new ItemOne {IDItem = 3, OneProperty = "3"},
                                new ItemOne {IDItem = 4, OneProperty = "4"}
                            };
            var list2 = new List<ItemTwo>
                            {
                                new ItemTwo {IDItem = 2, TwoProperty = "2"},
                                new ItemTwo {IDItem = 3, TwoProperty = "3"},
                            };

            var query = from x in list1
                        join y in list2 on x.IDItem equals y.IDItem
                            into z
                        from q in z.DefaultIfEmpty()
                        select new {IOne = x, ITwo = q};
            foreach (var pair in query)
            {
                if (pair.ITwo != null) // && pair.IOne.OneProperty != null
                    pair.IOne.OneProperty = pair.ITwo.TwoProperty;
            }

            var resultList = query.Select(x => x.IOne).ToList();

            foreach (var one in resultList)
            {
                Console.WriteLine(one.OneProperty);
            }
            System.Console.ReadLine();

        }

        public class ItemOne
        {
            public int IDItem { get; set; }
            public string OneProperty { get; set; }
        }

        public class ItemTwo
        {
            public int IDItem { get; set; }
            public string TwoProperty { get; set; }
        }
    }
}