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("驗證碼是：" + yan);      Console.Write("請輸入驗證碼：");      string c = Console.ReadLine();      if (c.ToUpper() == yan.ToUpper())      {        Console.WriteLine("驗證碼正確！");      }      else      {        Console.WriteLine("驗證碼錯誤！");      }      Console.ReadLine();