字符串

 截取生日
            Console.WriteLine("请输入身份证号：");
            string s = Convert.ToString(Console.ReadLine());
            s = s.Substring(6,8);
            Console.WriteLine("你的生日是："+s);

  生成四位随机数
            //机器生成
            Random rand = new Random();
            int n = rand.Next(10000);
            string s = n.ToString("0000");
            Console.Write(s);
             //人输入
            Console.WriteLine("请输入验证码：");
            string r = Console.ReadLine(); 

            //判断输出
            if (s == r)
            {
                Console.WriteLine("验证通过");
            }
            else 
           {
                Console.WriteLine("验证失败");
            }

Console.WriteLine("请输入邮箱：");
                string mail = Console.ReadLine();
                //转化为小写
                mail = mail.ToLower();
                //定义一个对比字符串
                string db = "abcdefghijklmnopqrstuvwxyz0123456789_@.";
                //定义变量a表示几个@，dian表示出现几个点，right表示输入正确
                int a = 0;
                int dian = 0;
                int right = 0;
                //判断输出第一个字符是否是_
                if (mail.First().ToString() == "_")
                {
                    Console.WriteLine("邮箱不能以_开头！");
                    right++;
                }
                //判断邮箱结尾是否以.com或.cn结尾
                if (mail.EndsWith(".com") || mail.EndsWith(".cn"))
                {

                }
                else
                {
                    Console.WriteLine("邮箱必须以.com或.cn结尾");
                    right++;
                }
                //逐个对比，看是否有特殊字符
                for (int i = 0; i < mail.Length; i++)
                {
                    string zf = mail.Substring(i, 1);
                    if (db.Contains(zf))
                    {
                        if (zf == "@")
                        {
                            a++;
                        }
                        if (zf == ".")
                        {
                            dian++;
                        }
                    }
                    else
                    {
                        Console.WriteLine("输入的邮箱含有特殊字符！");
                        right++;
                        break;
                    }
                }
                //判断点出现的个数是否是一个，判断@出现的个数是否是一个
                if (a == 1 && dian == 1)
                {
                    if (mail.Contains("@."))
                    {
                        Console.WriteLine("@和.之间必须又表示地址的字符！");
                        right++;
                    }
                    else
                    {
                        if (mail.IndexOf("@") > mail.IndexOf("."))
                        {
                            Console.WriteLine("@必须在.前面出现！");
                            right++;
                        }
                    }
                }
                else
                {
                    Console.WriteLine("@和.只能出现一次!");
                    right++;
                }
                //判断最终是否正确
                if (right == 0)
                {
                    Console.WriteLine("输入的邮箱格式正确");
                }
                else
                {
                    Console.WriteLine("邮箱有误");
                }

     //double n = Math.Pow(64,0.2);  //指数
            //double n = Math.Sqrt(9);  //平方根
            //int n = (int)Math.Ceiling(3.8);
            //double n = Math.Floor(3.8);

            double n = Math.Round(3.74);

            Console.WriteLine(n);

 int year = Convert.ToInt32(Console.ReadLine());
            int month = Convert.ToInt32(Console.ReadLine());
            int day = Convert.ToInt32(Console.ReadLine());
            //string s = Convert.ToString(DateTime.Now);
            //Console.WriteLine(s);

            try
            {
                DateTime dt = new DateTime(year, month, day);
                Console.WriteLine(dt);
            }
            catch
            {
                //Console.WriteLine("亲，日期输入不正确！");
            }
            //DateTime dt1 = new DateTime(1991, 11, 22,23,56,32);
            ////DateTime dt = new DateTime();
            //DateTime dt2 = DateTime.Now;
            ////Console.WriteLine(dt.ToString("yyyy年MM月dd日hh时mm分ss秒"));
            ////Console.WriteLine(dt.AddHours(-10000));
            //TimeSpan  tp = dt2 - dt1;
            //Console.WriteLine(tp.TotalDays);