using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static IEnumerable<T> ReplaceWhile<T>(IEnumerable<T> source, Func<T, bool> predicate, T subsitute)

    {

        var zerosLength = source.TakeWhile(predicate).Count();

        return Enumerable.Repeat(subsitute, zerosLength).Concat(source.Skip(zerosLength));

    }

    public static void Main()

    {

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

        Console.WriteLine(string.Join(",", ReplaceWhile(l, x => x == 0, (int?)null)));

    }

}