using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var l = new List<int?>{0, 0, 0, 0, 0, 0, 4, 5, 6, 3, 0, 0, 2, 3};

        var firstIndex = l.FindIndex(x => x != 0);

        var m = l.Select((item, index) => firstIndex == -1 || index < firstIndex ? null : item);

        Console.WriteLine(string.Join(',', m));

        l = new List<int?>{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

        firstIndex = l.FindIndex(x => x != 0);

        m = l.Select((item, index) => firstIndex == -1 || index < firstIndex ? null : item);

        Console.WriteLine(string.Join(',', m));

    }

}