c# XML省市联动

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

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
List<string> sheng = new List<string>();
List<string> shi = new List<string>();
List<string> qua = new List<string>();
private void Form1_Load(object sender, EventArgs e)
{
XmlDocument doc = new XmlDocument();
doc.Load("Provinces.xml");
comboBox1.Items.Clear();
sheng.Clear();
XmlNodeList nodelist = doc.SelectNodes("/Provinces/Province");

foreach (XmlNode item in nodelist)
{

comboBox1.Items.Add(item.Attributes["ProvinceName"].Value);
sheng.Add(item.Attributes["ID"].Value);

}
comboBox1.SelectedIndex = 0;

}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
XmlDocument xm = new XmlDocument();
xm.Load("Cities.xml");
shi.Clear();
comboBox2.Items.Clear();
string cid = sheng[comboBox1.SelectedIndex];
XmlNodeList citys = xm.SelectNodes("/Cities/City[@PID="+cid+"]");

foreach (XmlNode item in citys)
{

comboBox2.Items.Add(item.Attributes["CityName"].Value);
shi.Add(item.Attributes["ID"].Value);

}
comboBox2.SelectedIndex = 0;
}

private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
{

}

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
XmlDocument qu = new XmlDocument();
qu.Load("Districts.xml");
qua.Clear();
comboBox3.Items.Clear();
string qid = shi[comboBox2.SelectedIndex];
XmlNodeList qux = qu.SelectNodes("/Districts/District[@CID=" + qid + "]");
foreach (XmlNode item in qux)
{
comboBox3.Items.Add(item.Attributes["DistrictName"].Value);
qua.Add(item.Attributes["ID"].Value);
}
comboBox3.SelectedIndex = 0;
}
}
}