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();