string ss = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";      Random a = new Random();      int b;      string yan = "";      for (int i = 0; i < 4; i++)      {        b = a.Next(62);        yan += ss.Substring(b, 1);      }      Console.WriteLine("驗(yàn)證碼是：" + yan);      Console.Write("請(qǐng)輸入驗(yàn)證碼：");      string c = Console.ReadLine();      if (c.ToUpper() == yan.ToUpper())      {        Console.WriteLine("驗(yàn)證碼正確！");      }      else      {        Console.WriteLine("驗(yàn)證碼錯(cuò)誤！");      }      Console.ReadLine();