using System;

using System.Security.Cryptography;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(AccountNumberGenerator.GetAccountNumber());

    }

}

​

public static class AccountNumberGenerator

{

    public static string GetAccountNumber()

    {

        var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();

        var accountNumber = new System.Text.StringBuilder();

        var bytes = new byte[4];

        using (var csp = new RNGCryptoServiceProvider())

        {

            csp.GetBytes(bytes);

        }

        for (int i = 0; i < bytes.Length; i++)

        {

            // values 251-255 would give a slight bias to letters A-F

            while (bytes[i] > 250)

            {

                bytes[i] = GetRandomByte();

            }

            accountNumber.Append(chars[bytes[i] % 25]);

        }

        return accountNumber.ToString();

    }

    private static byte GetRandomByte() 

    {

        var bytes = new byte[1];

        using (var csp = new RNGCryptoServiceProvider())

        {

            csp.GetBytes(bytes);

        }

        return bytes[0];

    }

}