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

public class EmployeeDto
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class EmployeeEntity
{
    public int Id { get; set; }
    public string FullName { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        List<object> o = new List<object>();
        o.Add(new EmployeeDto { Id = 1, Name = "Bob" });
        o.Add(null);
        o.Add(new EmployeeDto { Id = 2, Name = "Foo" });
        o.Add(null);
        o.AddRange(AddMilionOfEmployees());

        Stopwatch sw = new Stopwatch();
        sw.Start();
        var employeeEntityList = MapList1(o);
        sw.Stop();
        Console.WriteLine("ElapsedTicks Solution 1 (OfType): " + sw.ElapsedTicks);

        //Console.WriteLine("--");

        //sw = new Stopwatch();
        //sw.Start();
        //var employeeEntityList2 = MapList2(o);
        //sw.Stop();
        //Console.WriteLine(sw.ElapsedTicks);

        Console.ReadKey();
    }

    private static IEnumerable<object> AddMilionOfEmployees()
    {
        for (int i = 0; i < 100000; i++) //ideone doesn't allow a million.. ¬¬
        {
            Random randNum = new Random();
            yield return new EmployeeDto { Id = i, Name = String.Format("Employee{0}", i * randNum.Next()) };
        }
    }

    private static IEnumerable<EmployeeEntity> MapList1(List<object> o)
    {
        return
            from i in o.OfType<EmployeeDto>()
            select new EmployeeEntity
            {
                Id = i.Id,
                FullName = i.Name
            };
    }

    private static IEnumerable<EmployeeEntity> MapList2(List<object> o)
    {
        return
            from si in o
            let i = si as EmployeeDto
            where i != null
            select new EmployeeEntity
            {
                Id = i.Id,
                FullName = i.Name
            };
    }

}