类的例题

Console.Write("请输入身份证号：");
            string id = Console.ReadLine();
            if (id.Length == 18)
            {
                string year = id.Substring(6, 4);
                string month = id.Substring(10, 2);
                string day = id.Substring(12, 2);
                Console.WriteLine("您的生日是：" + year + "年" + month + "月" + day + "日");
            }
            else
            {
                Console.WriteLine("输入有误！");
            }
            Console.ReadLine();

Console.Write("请输入你的邮箱账号：");
            string mail = Console.ReadLine();
            bool a=mail.Contains("@");//有且只能有一个@
            if(a==true)
            {
                int b=mail.IndexOf ("@");//从前面找@
                int c=mail.LastIndexOf("@");//从后面找@
                if(b==c)//如果相等那么就有一个@
                {
                    if (b != 0)//索引号不等于0 那@不是开头
                    {
                        string mail1 = mail.Substring(b);//从@开始往后截取为mail1
                        if (mail1.Contains("."))//是否包含此字符串“.”
                        {
                            int d = mail1.IndexOf(".");//@之后至少有一个
                            if (d != 1)//@和“.”不能靠在一起
                            {
                                int e = mail1.LastIndexOf(".");
                                if (e != mail1.Length - 1)//“.”不能是最后一位
                                {
                                    Console.WriteLine("邮箱格式输入正确！");
                                }
                                else
                                {
                                    Console.WriteLine("输入有误！");
                                }
                            }
                            else
                            {
                                Console.WriteLine("输入有误！");
                            }
                        }
                        else
                        {
                            Console.WriteLine("输入有误！");
                        }
                    }
                    else
                    {
                        Console.WriteLine("输入有误。");
                    }
                }
                else 
                {
                    Console.WriteLine("输入有误！");
                }
            }
            else
            {
                Console.WriteLine("输入有误！");
            }
           Console.ReadLine();

string s = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
            for (; ; )
            {
            Random ran = new Random();
            int a = ran.Next(s.Length-1);
            int b = ran.Next(s.Length-1);
            int c = ran.Next(s.Length-1);
            int d = ran.Next(s.Length-1);
            string e = (s.Substring(a, 1));
            string f = (s.Substring(b, 1));
            string g = (s.Substring(c, 1));
            string h=(s.Substring(d, 1));
            Console.WriteLine(e+f+g+h);       
            
            Console.Write("请输入验证码：");
            string m = Console.ReadLine();
            m = m.ToLower();
            e = e.ToLower();
            f = f.ToLower();
            g = g.ToLower();
            h = h.ToLower();     
       
                if ((e == m.Substring(0, 1)) && (f == m.Substring(1, 1)) && (g == m.Substring(2, 1)) && (h == m.Substring(3, 1)))
                {
                    Console.Write("输入正确");
                    break;
                }
                else
                {
                    Console.WriteLine("您输入的验证码有误！请从新输入");
                }
                Console.ReadLine();
                Console.Clear();               
            }

 string s = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
            Random ran = new Random();
            for (; ; )
            {
                string a = "";
                for (int i = 1; i <= 4; i++)
                {
                    a += s.Substring(ran.Next(s.Length), 1);
                }
                Console.WriteLine(a);

                Console.WriteLine("请输入验证码：");
                string b = Console.ReadLine();
                if (b.ToLower() == a.ToLower())
                {
                    Console.WriteLine("输入正确");
                    break;
                }
                else
                {
                    Console.Clear();
                    Console.WriteLine("输入错误");
                }
            }
            Console.ReadLine();