using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Test> lstTest = new List<Test>();

        lstTest.Add(new Test() { Id = 1, Name = "Test 1", CreatedDate = Convert.ToDateTime("05/05/2005"), ModifiedDate = Convert.ToDateTime("09/05/2005") });

        lstTest.Add(new Test() { Id = 2, Name = "Test 2", CreatedDate = Convert.ToDateTime("06/05/2005"), ModifiedDate = Convert.ToDateTime("07/05/2005") });

        lstTest.Add(new Test() { Id = 3, Name = "Test 3", CreatedDate = Convert.ToDateTime("08/05/2005"), ModifiedDate = null });

        lstTest.Add(new Test() { Id = 4, Name = "Test 4", CreatedDate = Convert.ToDateTime("10/05/2005"), ModifiedDate = null });

        Test lastEntry = lstTest

            .OrderBy(t => t.ModifiedDate ?? t.CreatedDate)

            .LastOrDefault();

        Console.WriteLine(lastEntry.Name);

    }

}

​

public class Test

{

        public int Id { get; set; }

        public string Name { get; set; }

        public DateTime CreatedDate { get; set; }

        public DateTime? ModifiedDate { get; set; }

}