https://salesforce.stackexchange.com/questions/173224 in C#

UniqueNumberGenerator.cs

namespace NumberGenerators {
    public class UniqueNumberGenerator
    {
        int value;
        public UniqueNumberGenerator(int value)
        {
            this.value = value;
        }

        public Nullable<int> NextId()
        {
            value++;
            int a = (value >> 25) & 31, b = (value >> 20) & 31, c = (value >> 15) & 31,
                        d = (value >> 10) & 31, e = (value >> 5) & 31, f = value & 31;
            for (; a < 8; b = 8, a++)
            {
                for (; b < 32; c = 0, b++)
                {
                    if (a != b)
                    {
                        for (; c < 32; d = 0, c++)
                        {
                            if (a != c && b != c)
                            {
                                for (; d < 32; e = 0, d++)
                                {
                                    if (a != d && b != d && c != d)
                                    {
                                        for (; e < 32; f = 0, e++)
                                        {
                                            if (a != e && b != e && c != e && d != e)
                                            {
                                                for (; f < 32; f++)
                                                {
                                                    if (a != f && b != f && c != f && d != f && e != f)
                                                    {
                                                        value = (a << 25) + (b << 20) + (c << 15) + (d << 10) + (e << 5) + f;
                                                        return value;
                                                    }
                                                }
                                            }
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            }
            return null;
        }
        string keys = "23456789ABCDEFGHJKLMNPQRSTUVWXYZ";

        public string Base32encode() => keys.Substring((value >> 25) & 31,1) + keys.Substring((value >> 20) & 31,1) + 
            keys.Substring((value >> 15) & 31,1) + keys.Substring((value >> 10) & 31,1) + 
            keys.Substring((value >> 5) & 31,1) + keys.Substring(value & 31,1);
    }
}