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("验证码是：" + yan);
      Console.Write("请输入验证码：");
      string c = Console.ReadLine();
      if (c.ToUpper() == yan.ToUpper())
      {
        Console.WriteLine("验证码正确！");
      }
      else
      {
        Console.WriteLine("验证码错误！");
      }
      Console.ReadLine();