HQ-day7 随机数案例：随机出验证码，对照输入，判断是否正确

string s = "abcdefghiklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
            for (; ; )
            {
                Random ran = new Random();
                int a = ran.Next(s.Length);   //随机抽取s内的字符a,b,c,d
                int b = ran.Next(s.Length);
                int c = ran.Next(s.Length);
                int d = ran.Next(s.Length);

                string a1 = s.Substring(a, 1);//接收a,b,c,d的字符串a1,b1,c1,d1
                string b1 = s.Substring(b, 1);
                string c1 = s.Substring(c, 1);
                string d1 = s.Substring(d, 1);
                string ss = a1 + b1 + c1 + d1;//组成验证码
                Console.WriteLine(ss); //输出验证码

                Console.Write("请输入验证码：");//用户输入验证码
                string yzm = Console.ReadLine();

                string ss1 = ss.ToUpper();  //将输入的验证码全部转化为大写字符（或者小写）
                string yzm1 = yzm.ToUpper();

                if (ss1 == yzm1)   //比对输入和显示是否一致
                {
                    Console.WriteLine("输入正确");
                    break;

                }
                else
                {
                    Console.WriteLine("您输入的有误！");
                }
            }
            Console.ReadLine();

 简化代码：

  string s = "abcdefghiklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
             for (; ; )
                {
                    Random ran = new Random();
               
                    string biao = "";
                    for (int i = 1; i <= 4; i++)
                    {
                        biao += s.Substring(ran.Next(s.Length), 1);
                    }
                    Console.WriteLine(biao);
                    Console.Write("请输入验证码");
                    string shu = Console.ReadLine();
                    if (shu.ToUpper() == biao.ToUpper())
                    {
                        Console.WriteLine("输入正确");
                        break;

                    }

                    else
                    {
                        Console.Clear();
                        Console.WriteLine("输入错误");
                        
                    }

                }