.NET Fiddle

CSharp · Console

/Program.cs

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);
		}
	
	}
}