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();