using System;

using System.Collections.Generic;

public class Program

{

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

    {

        var matching = true;

        foreach(var item in source)

            yield return !matching ? item : (matching = predicate(item)) ? subsitute : item;

    }

    public static void Main()

    {

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

        var e = ReplaceWhile(l, x => x == 0, (int?)null);

        Console.WriteLine(string.Join(",", e));

        Console.WriteLine(string.Join(",", e));

    }

}