using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

public class Test
{
	public static void Main()
	{
        Random r = new Random(DateTime.Now.Millisecond);
        Dictionary<Char, Int32> seed = Enumerable.Range(65, 26).ToDictionary(x => (Char)x, x => r.Next(10) % 5 != 0 ? r.Next(65536) : 1);
        List<Char> sample = new List<Char>();
        foreach (KeyValuePair<Char, Int32> kvp in seed)
        {
            for (Int32 i = 0; i < kvp.Value; i++) 
            {
                sample.Add(kvp.Key);
            }
        }
    
        Stopwatch sw = new Stopwatch();
        sw.Start();
        var result = sample.GroupBy (x => x).Where(x => x.Count() == 1).Select (x => x.Key);
        sw.Stop();
    
        Console.WriteLine(String.Format("{0} to go through {1} records.", sw.Elapsed, sample.Count));
        Console.WriteLine(String.Format("Result: {0}", String.Join(", ", result.Select(x => x.ToString()).ToArray())));
	}
}