C#中comboBox实现三级联动

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

namespace Select
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }
    Hashtable province = new Hashtable();
    Hashtable city = new Hashtable();
    private void Province()
    {
      province.Add("云南省",new string[] {"昆明市","玉溪市" });
      province.Add("四川省",new string[] { "成都市","绵阳市" });
      city.Add("昆明市",new string[] {"盘龙区","五华区" });
      city.Add("玉溪市",new string[] {"红塔区","。。。区" });
      city.Add("成都市",new string[] { "。。。区","。。。区" });
      city.Add("绵阳市",new string[] { "...区","...区" });
    }

    private void Form1_Load(object sender,EventArgs e)
    {
      Province();
      foreach (string str in province.Keys)
      {
        comboBox1.Items.Add(str);
      }
      foreach (string str in city.Keys)
      {
        comboBox2.Items.Add(str);
      }
      comboBox1.SelectedIndex=0;
    }

    private void comboBox1_SelectedIndexChanged(object sender,EventArgs e)
    {
      string[] citys = province[comboBox1.Text] as string[];
      comboBox2.Items.Clear();
      foreach (string s in citys)
      {
        comboBox2.Items.Add(s);
      } 
      comboBox2.SelectedIndex = 0;
    }
    private void comboBox2_SelectedIndexChanged(object sender,EventArgs e)
    {
      string[] citys = city[comboBox2.Text] as string[];
      comboBox3.Items.Clear();
      foreach (string str in citys)
      {
        comboBox3.Items.Add(str);
      }
      comboBox3.SelectedIndex = 0;
    }

    private void comboBox3_SelectedIndexChanged(object sender,EventArgs e)
    {

    }
  }
}