When random is too consistent...

A while back I posted an extension method for shuffling Lists in C# using the Fisher Yates Shuffle. The algorithm is valid but my implementation relied on the default random number generator in .Net (System.Random) which is flawed since it generates a predictable sort of random.

Here's a correction that uses a more random sort of random from System.Security.Cryptography:

public static void Shuffle<T>(this IList<T> list)
{
    var provider = new RNGCryptoServiceProvider();
    int n = list.Count;
    while (n > 1)
    {
        var box = new byte[1];
        do provider.GetBytes(box);
        while (!(box[0] < n * (Byte.MaxValue / n)));
        var k = (box[0] % n);
        n--;
        var value = list[k];
        list[k] = list[n];
        list[n] = value;
    }
}

Here's a simple test you can run to demonstrate the difference in the algorithms:

using System;
using System.Collections.Generic;
using System.Security.Cryptography;

namespace AlphabetSoup
{
    class Program
    {
        static void Main()
        {
            var alphabet = GetAlphabet();
            foreach (var c in alphabet)
                Console.Write(c);
            Console.Write('\n');
            Console.Write('\n');

            Console.WriteLine("System.Random:");
            for (int i = 0; i < 10; i++)
            {
                alphabet = GetAlphabet();
                alphabet.QuickShuffle();
                foreach (var c in alphabet)
                    Console.Write(c);
                Console.Write('\n');
            }
            Console.Write('\n');
            Console.Write('\n');

            Console.WriteLine("System.Security.Cryptography:");
            for (int i = 0; i < 10; i++)
            {
                alphabet = GetAlphabet();
                alphabet.Shuffle();
                foreach (var c in alphabet)
                    Console.Write(c);
                Console.Write('\n');
            }
            Console.Write('\n');
            Console.Write('\n');

            Console.ReadKey();
        }

        static List<char> GetAlphabet()
        {
            var alphabet = new List<char>();
            for (int c = 65; c < 91; c++)
                alphabet.Add((char)c);
            return alphabet;
        }
    }
    static class Ext
    {
        public static void Shuffle<T>(this IList<T> list)
        {
            var provider = new RNGCryptoServiceProvider();
            int n = list.Count;
            while (n > 1)
            {
                var box = new byte[1];
                do provider.GetBytes(box);
                while (!(box[0] < n * (Byte.MaxValue / n)));
                var k = (box[0] % n);
                n--;
                var value = list[k];
                list[k] = list[n];
                list[n] = value;
            }
        }
        public static void QuickShuffle<T>(this IList<T> list)
        {
            var rng = new Random();
            var n = list.Count;
            while (n > 1)
            {
                n--;
                var k = rng.Next(n + 1);
                var value = list[k];
                list[k] = list[n];
                list[n] = value;
            }
        }
    }
}

Here's the output on my machine: