IotaSeedGenerator, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Linq;
using System.Security.Cryptography;
using System.Text;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var validCharacters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ9".ToList();
            var len = 81;
            var cnt = 10;
            using (var rng = new RNGCryptoServiceProvider())
            {
                while (--cnt > 0)
                {
                    var sb = new StringBuilder(len);
                    while (sb.Length < len)
                    {
                        var nextUint = rng.GetNextUint();
                        var nextIndex = (int) (nextUint % validCharacters.Count);
                        var nextCharacter = validCharacters[nextIndex];

if (validCharacters.Contains(nextCharacter)) sb.Append(nextCharacter);
                    }
                    Console.WriteLine(sb);
                }
            }
        }
    }

public static class RngExt
    {
        public static uint GetNextUint(this RandomNumberGenerator r)
        {
            var buffer = new byte[sizeof(uint)];
            r.GetBytes(buffer);
            return BitConverter.ToUInt32(buffer, 0);
        }
    }
}

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Compilation time: 0,14 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 12 Mb, average nr of threads: 3