asp.net生成数字、字母随机数

 char[] constant = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' };
                StringBuilder ascii = new StringBuilder();
               // byte[] bytes = new byte[100];
                Random random = new Random();
               
              //  int code;
               // string strRnd = "游客" + random.Next(9).ToString() + random.Next(9).ToString() + random.Next(9).ToString();
                for (int i = 0; i < 4; i++)
                {
                   // code = random.Next(62);
                   ascii.Append(constant[random.Next(62)]);
                }
               // ASCIIEncoding ascii = new ASCIIEncoding();
                lbl.Text = ascii.ToString();