using System;

using System.Collections;   

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var l = new List<int?>(new int?[]{null, 1, 2, 3, null, 2, 4, null, 5, 6});

        var l2 = from e in l

            orderby e.HasValue descending, e

            select e;

        Console.WriteLine(string.Join(" - " , l2));

    }

}