using System;

using System.Collections.Generic;

using System.Linq;

using System.Globalization;

using System.Text.RegularExpressions;

​

public class record

{

    public int flashid { get; set; }

    public int position {get; set; }

    public string value { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        var list = new List<record>

        {

            new record{ flashid = 450, position = 5, value = "a"},

            new record{ flashid = 450, position = 6, value = "b"},

            new record{ flashid = 450, position = 7, value = "c"},

            new record{ flashid = 450, position = 7, value = "d"},

            new record{ flashid = 450, position = 7, value = "e"},

            new record{ flashid = 450, position = 8, value = "f"},

            new record{ flashid = 450, position = 9, value = "g"}

        };

        var extendedList = Enumerable

            .Range(1, 10)

            .GroupJoin(list, n => n, r => r.position, (n, g) => g

                .DefaultIfEmpty(new record { flashid = 450, position = n, value = "." }));

        var result = extendedList

            .Aggregate(Enumerable.Repeat("", 1), (a, c) => a

                .SelectMany(b => c

                    .Select(d => b + d.value)));

        foreach (var r in result)

        {

            Console.WriteLine(r);

        }

    }

}