using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        int?[] a = new int?[] { 2, null, 1, null, 3 };

​

        a = a

            .OrderBy(x => !x.HasValue)

            .ThenBy(x => x)

            .ToArray();

​

        foreach (var i in a)

        {

            Console.WriteLine(i == null ? "NULL" : i.ToString());

        }

    }

}