iLength---生成的隨機數的位數
public static string GetRandomString(int iLength)
    {
         string buffer = "0123456789";// 隨機字元中也可以為漢字（任何）  
         StringBuilder sb = new StringBuilder();
         Random r = new Random();
         int range = buffer.Length;
         for (int i = 0; i < iLength; i++)
         {
             sb.Append(buffer.Substring(r.Next(range), 1));
         }
            return sb.ToString();
    }