在User类中添加两个私有的成员变量，用来表示生日和年龄，并设计相应的属性完成对其的访问，要求生日是只写属性，并能判断出如果用户输入的生日年份小于1900年，或当前年龄小于3岁，给出输入错误提示信息，

public class User
       {
           private string name;
           private string sex;
           private DateTime birthday;
           private int age;
           public string Name
           {
               get { return name; }
               set { name = value; }
           }
           public string Sex
           {
               get { return sex; }
               set
               {
                   if (value == "男" || value == "女")
                   {
                       sex = value;
                   }
                   else
                   {
                       Console.WriteLine("乖乖输入不合适哦");
                   }
               }
           }
           public DateTime Birthday
           {
               set
               {
                   if (value.Year <= 1900 || DateTime.Now.Year - value.Year <= 3)
                   {
                       Console.WriteLine("乖乖输入不合理哦");
                   }
                   else
                   {
                       birthday = value;
                   }
               }
           }
             public int Age
             {
                 get
                 {
                     age=DateTime .Now.Year-birthday.Year;
                     return age;
                 }
             }
       }
        class Program
        {
            static void Main(string[] args)
            {
               User sheep=new User();
                sheep.Name="糕羊";
                sheep.Sex="女";
                sheep.Birthday=Convert.ToDateTime("2003-1-1");
                Console.WriteLine("姓名："+sheep.Name+"\n性别："+sheep.Sex+"\n年龄："+sheep.Age);
                Console.ReadLine();

            }
        }