class Program
    {
        class testClass
        {
            public int Id { get; set; }
            public string Name { get; set; }
        }

        static void Main(string[] args)
        {

            var listOne = new List<testClass>();
            var listTwo = new List<testClass>();

            listOne.Add(new testClass { Id = 1, Name = "Test1" });
            listOne.Add(new testClass { Id = 2, Name = "Test2" });
            listOne.Add(new testClass { Id = 3, Name = "Test3" });
            listOne.Add(new testClass { Id = 4, Name = "Test4" });

            listTwo.Add(new testClass { Id = 1, Name = null });
            listTwo.Add(new testClass { Id = 2, Name = null });
            listTwo.Add(new testClass { Id = 3, Name = null });
            listTwo.Add(new testClass { Id = 4, Name = null });


            foreach (var item in listTwo)
            {
                item.Name = listOne.FirstOrDefault(x => x.Id == item.Id)?.Name;
            }

            Console.ReadKey();
        }

    }