改进~！
using System;
using System.Windows.Forms;
class shuxing
{
    private string m_name;
    private string m_sex;
    private DateTime m_birthday;
    public DateTime Birthday  //只写属性
    {
        set
        {
            if(value<Convert.ToDateTime("1900-1-1")||value.Year>DateTime.Now.Year-3)//取年份后为int类型？ 可用-运算符
                MessageBox.Show("输入非法");
            else 
                m_birthday=value;
        }
    }
    public int Age  //只读属性   返回int类型
    {
        get
        {
            return DateTime.Now.Year-m_birthday.Year;
        }
    }
    public string Name//读写属性
    {
        set
        {
            m_name=value;
        }
        get
        {
            return m_name;
        }
    }
    public string Sex//读写属性
    {
        set
        {
            if(value=="男"||value=="女")
                m_sex=value;
            else
                Console.WriteLine("性别必须为男或女");
        }
        get
        {
            return m_sex;
        }
    }
}
class text
{
    static void Main()
    {
        shuxing qq=new shuxing();
        qq.Name="李纹";
        qq.Sex="女";
        qq.Birthday=Convert.ToDateTime("2004-1-1");//Birthday为DateTime类型，所以要用to转换
        Console.WriteLine("姓名   "+qq.Name+"  性别  "+qq.Sex+"  年龄  "+qq.Age);
        
    }
}