var rand = new Random();
            string str = "0";
            const string chars = "ABCDEF0123456789";
            str += new string(Enumerable.Repeat(chars, 31)
               .Select(s => s[rand.Next(s.Length)]).ToArray());
            return str;

var rand = new Random();
            string str = "0";
            const string chars = "ABCDEF0123456789";
            str += new string(Enumerable.Repeat(chars, 31)
               .Select(s => s[rand.Next(s.Length)]).ToArray());
            return str;