一个表三级联动---WinFrom

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
namespace rr
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void Fill1()
        {
            List<ChinaStates> list = new ChinaStatesDA().Select("0001");
            comboBox1.DataSource = list;
            comboBox1.DisplayMember = "AreaName";
            comboBox1.ValueMember = "AreaCode";
        }
        private void Fill2()
        {
 
            string parentcode="";
            if (comboBox1.SelectedItem!=null)
            {
                            parentcode = (comboBox1.SelectedItem as ChinaStates).AreaCode;
 
            }
 
            List<ChinaStates> list = new ChinaStatesDA().Select(parentcode);
            comboBox2.DataSource = list;
            comboBox2.DisplayMember = "AreaName";
            comboBox2.ValueMember = "AreaCode";
 
        }
        private void Fill3()
        {
            string parentcode = "";
            if (comboBox2.SelectedItem != null)
            {
                parentcode = (comboBox2.SelectedItem as ChinaStates).AreaCode;
 
            }
            List<ChinaStates> list = new ChinaStatesDA().Select(parentcode);
            comboBox3.DataSource = list;
            comboBox3.DisplayMember = "AreaName";
            comboBox3.ValueMember = "AreaCode";
 
        }
        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
 
            Fill3();
        }
 
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            Fill2();
            Fill3();
        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
            Fill1();
            Fill2();
            Fill3();
        }
    }
    class DBcon
    {
        public const string CONSTR = "server=.;database=yy;uid=sa;pwd=111111";
    }
    class ChinaStates
    {
        public string AreaCode { get; set; }
        public string AreaName { get; set; }
        public string ParentCode { get; set; }
    }
    class ChinaStatesDA
    {
        private SqlConnection Con;
        private SqlCommand Cmd;
        private SqlDataReader dr;
        public ChinaStatesDA()
        {
            Con = new SqlConnection(DBcon.CONSTR);
            Cmd = Con.CreateCommand();
        }
        public List<ChinaStates> Select(string code)
        {
            List<ChinaStates> list = new List<ChinaStates>();
            Cmd.CommandText = "select * from ChinaStates where ParentAreaCode=@code";
            Cmd.Parameters.Clear();
            Cmd.Parameters.AddWithValue("@code", code);
            try
            {
                Con.Open();
                dr = Cmd.ExecuteReader();
                while (dr.Read())
                {
                    ChinaStates data = new ChinaStates();
                    data.AreaCode = dr["AreaCode"].ToString();
                    data.AreaName = dr["AreaName"].ToString();
                    data.ParentCode = dr["ParentAreaCode"].ToString();
                    list.Add(data);
                }
            }
            finally
            {
                Con.Close();
            }
 
 
            return list;
        }
    }
}