练习：WinForm 三级联动（中国行政区划）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace 三级联动.Model
{
    class China
    {
        private string code;

        public string Code
        {
            get { return code; }
            set { code = value; }
        }
        private string name;

        public string Name
        {
            get { return name; }
            set { name = value; }
        }
        private string parentcode;

        public string Parentcode
        {
            get { return parentcode; }
            set { parentcode = value; }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.SqlClient;

namespace 三级联动.DataConnection
{
    class DataConnection
    {
        private static string connstr="server=.; database=mydb; user=sa; pwd=ray;";
        public static SqlConnection Conn
        {
            get{return new SqlConnection(connstr); }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.SqlClient;

namespace 三级联动.DataOperation
{
    class ChinaData
    {
        private SqlConnection _conn;
        private SqlCommand _cmd;
        private SqlDataReader _dr;

        public ChinaData()
        {
            _conn = DataConnection.DataConnection.Conn;
            _cmd = _conn.CreateCommand();
        }

        //根据父级代号返回地区信息
        public List<Model.China> Select(string pcode)
        {
            _cmd.CommandText = "select *from ChinaStates where ParentAreaCode=@pcode";
            _cmd.Parameters.Clear();
            _cmd.Parameters.AddWithValue("@pcode",pcode);
            _conn.Open();
            _dr = _cmd.ExecuteReader();
            List<Model.China> list = new List<Model.China>();

            if (_dr.HasRows)
            {
                while (_dr.Read())
                {
                    Model.China data = new Model.China();
                    data.Code = _dr[0].ToString();
                    data.Name = _dr[1].ToString();
                    data.Parentcode = _dr[2].ToString();

                    list.Add(data);
                }
            }
            _conn.Close();
            return list;
        }
    }
}

可视化界面操作：

后台代码：

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;

namespace 三级联动
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        DataOperation.ChinaData cda = new DataOperation.ChinaData();

        //填充省的方法
        public void FillSheng()
        {
            List<Model.China> list = cda.Select("0001");
            cmbSheng.DataSource = list;
            cmbSheng.DisplayMember = "Name";
            cmbSheng.ValueMember = "Code";
        }

        //填充市的方法
        public void FillShi()
        {
            List<Model.China> list = cda.Select(cmbSheng.SelectedValue.ToString());
            cmbShi.DataSource = list;
            cmbShi.DisplayMember = "Name";
            cmbShi.ValueMember = "Code";
        }

        //填充区的方法
        public void FillQu()
        {
            List<Model.China> list = cda.Select(cmbShi.SelectedValue.ToString());
            cmbQu.DataSource = list;
            cmbQu.DisplayMember = "Name";
            cmbQu.ValueMember = "Code";
        }

        //页面一加载运行三个方法
        private void Form1_Load(object sender, EventArgs e)
        {
            FillSheng();
            FillShi();
            FillQu();
        }

        //填充市的下拉列表
        private void cmbSheng_SelectionChangeCommitted(object sender, EventArgs e)
        {
            FillShi();
            FillQu();
        }

        //填充区的下拉列表
        private void cmbShi_SelectionChangeCommitted(object sender, EventArgs e)
        {
            FillQu();
        }
    }
}