public class Student
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
        public char Gender { get; set; }

    }

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Xml;

namespace XML_Login
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        List<Student> list = new List<Student>();

        //1创建xml对象
        XmlDocument doc = new XmlDocument();
        private void Form1_Load(object sender, EventArgs e)
        {

        }

        #region 01创建文件

        public void CreateFile()
        {
            //1创建声明信息
            XmlDeclaration dec = doc.CreateXmlDeclaration("1.0", "utf-8", null);
            doc.AppendChild(dec);
            //2创建person根节点
            XmlElement person = doc.CreateElement("Person");
            doc.AppendChild(person);
            AddNode(person);
        } 
        #endregion

        #region  03添加节点
        private void AddNode(XmlElement person) 
        {
            //3创建Student子节点
            XmlElement student = doc.CreateElement("Student");
            person.AppendChild(student);
            //4Student子节点设置属性Id
            student.SetAttribute("ID", txtID.Text.Trim());
            //5Student添加Name节点
            XmlElement name = doc.CreateElement("Name");
            name.InnerText = txtName.Text.Trim();
            student.AppendChild(name);
            //5Student添加Age节点
            XmlElement age = doc.CreateElement("Age");
            age.InnerText = txtAge.Text.Trim();
            student.AppendChild(age);
            //6Student添加Gender节点
            XmlElement gender = doc.CreateElement("Gender");
            if (RDMan.Checked == true)
            {
                gender.InnerText = "男";
            }
            else
            {
                gender.InnerText = "女";
            }
            student.AppendChild(gender);
        }
        #endregion

        #region 04判断studentID是否存在
        public bool IsExist() 
        {
            doc.Load("Person.xml");
            //1 获取根节点
            XmlElement person = doc.DocumentElement;
            //2 获取所有student的属性
            XmlNodeList students = person.ChildNodes;
            List<string> listID = new List<string>();
            foreach (XmlNode item in students)
            {
                listID.Add(item.Attributes["ID"].Value);
            }
            if (listID.Contains(txtID.Text))
            {                
                return true;
            }
            return false;
        }
        #endregion

        #region 04"添加"按钮触发事件
        private void btnAdd_Click(object sender, EventArgs e)
        {
            int Id;
            if (!int.TryParse(txtID.Text.Trim(), out Id))
            {
                MessageBox.Show("ID请输入正整数");
                return;
            }
            if (File.Exists("Person.xml"))
            {
                //判断student ID是否存在
                if (IsExist())
                {
                    MessageBox.Show("ID已存在");
                    return;
                }
                //添加节点                
                doc.Load("Person.xml");
                XmlElement person = doc.DocumentElement;
                AddNode(person);
            }
            else
            {
                //创建文件--同时把当前页面的Student信息保存到文件中
                CreateFile();
            }
            doc.Save("Person.xml");
            MessageBox.Show("保存成功!");
        } 
        #endregion
    }
}