public static string CreateShortCode(int size = 6)
        {
            var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
            var guid = Create();

            var result = new StringBuilder(size);
            foreach (var b in guid.ToByteArray().Take(size))
            {
                result.Append(chars[b % chars.Length]);
            }

            return result.ToString();
        }

public static string CreateShortCode(int size = 6)
        {
            var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
            var maxRandomValue = (256 / chars.Length) * chars.Length;

            var result = new StringBuilder(size);
            while (result.Length < size)
            {
                var data = new byte[size];
                using (var rng = RandomNumberGenerator.Create())
                {
                    rng.GetBytes(data);
                }

                foreach (var b in data)
                {
                    if (b > maxRandomValue)
                    {
                        continue;
                    }

                    result.Append(chars[b % chars.Length]);
                }
            }

            return result.ToString();
        }