C# 二级下拉列表框联动(ComboBox)(ExecuteReader)

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using System;using System.Collections.Generic;using System.ComponentModel;using System.Configuration;using System.Data;using System.Data.SqlClient;using System.Drawing;using System.IO;using System.Linq;using System.Text;using System.Threading.Tasks;using System.Windows.Forms;namespace _02ProvinceCitySelect{    public partial class MainFrm : Form    {        public MainFrm()        {            InitializeComponent();        }        private void MainFrm_Load(object sender, EventArgs e)        {            //加载数据库中的所有的省的数据            string connStr = ConfigurationManager.ConnectionStrings["sqlConn"].ConnectionString;            //创建链接对象            using (SqlConnection conn =new SqlConnection(connStr))            {                using (SqlCommand cmd =conn.CreateCommand())                {                    conn.Open();//***********8                    cmd.CommandText =@"select AreaId, AreaName, AreaPid from [dbo].[AreaFull] where AreaPId=0";                    using (SqlDataReader reader = cmd.ExecuteReader())                    {                        while ( reader.Read())                        {                            //int AreadId = int.Parse(reader["AreaId"].ToString());                            //把表格的数据转换成 对象数据                            AreaInfo areaInfo = new AreaInfo();  //AreaInfo是自定义的省城市信息类，重写其中的ToString()方法。                            areaInfo.AreaId = int.Parse(reader["AreaId"].ToString());                            areaInfo.AreaName = reader["AreaName"].ToString();                            areaInfo.AreaPId = int.Parse(reader["AreaPId"].ToString());                            //把省的信息放到 ComboBox中。ComboBox显示信息是 Item对象的ToString（）。下拉列表项本质上是一个对象。                            this.cbxProvince.Items.Add(areaInfo);  //自动调用areaInfo对象的ToString()方法。自定义类中重写ToString()方法                        }                    }//end useing  reader                }//end  useing cmd            }//end  using conn            this.cbxProvince.SelectedIndex = 0;        }        private void cbxProvince_SelectedIndexChanged(object sender, EventArgs e)  //当第一级下拉列表改变选项时(改变省份)        {            AreaInfo provinceAreaInfo= this.cbxProvince.SelectedItem as AreaInfo;  //下拉列表项本质上是一个对象，转成对象然后取出对象中的Id。根据Id填充第二级下拉列表框的项。            //判断是否 拿到的城市为空            if (provinceAreaInfo == null)            {                return;            }            //根据省的Id获取所有的城市信息            //加载数据库中的所有的省的数据            string connStr = ConfigurationManager.ConnectionStrings["sqlConn"].ConnectionString;            //创建链接对象            using (SqlConnection conn = new SqlConnection(connStr))            {                using (SqlCommand cmd = conn.CreateCommand())                {                    conn.Open();//***********8                    cmd.CommandText = @"select AreaId, AreaName, AreaPid from [dbo].[AreaFull] where AreaPId="+provinceAreaInfo.AreaId; //根据省份的Id查询城市。                    using (SqlDataReader reader = cmd.ExecuteReader())                    {                        this.cbxCity.Items.Clear();                        while (reader.Read())                        {                            //int AreadId = int.Parse(reader["AreaId"].ToString());                            //把表格的数据转换成 对象数据                            AreaInfo areaInfo = new AreaInfo();                            areaInfo.AreaId = int.Parse(reader["AreaId"].ToString());                            areaInfo.AreaName = reader["AreaName"].ToString();                            areaInfo.AreaPId = int.Parse(reader["AreaPId"].ToString());                            //把省的信息放到 ComboBox中。ComboBox显示信息是 Item对象的ToString（）                            this.cbxCity.Items.Add(areaInfo);                        }                    }//end useing  reader                }//end  useing cmd            }//en            this.cbxCity.SelectedIndex = 0;        }        private void btbExport_Click(object sender, EventArgs e)        {            #region 选择保存文件            string fileName = string.Empty;// 保存的文件名            //让用户选择 要保存的文件路径            using (SaveFileDialog sfd = new SaveFileDialog())            {                if (sfd.ShowDialog() != DialogResult.OK)                {                    return;                }                fileName = sfd.FileName;            }             #endregion            // 查询数据，写入数据，写入文本文件            string connStr = ConfigurationManager.ConnectionStrings["sqlConn"].ConnectionString;            using (SqlConnection conn =new SqlConnection(connStr))            {                using (SqlCommand cmd =conn.CreateCommand())                {                    conn.Open();                    cmd.CommandText = "select AreaId, AreaName, AreaPid from [dbo].[AreaFull]";                    using (SqlDataReader reader =cmd.ExecuteReader())                    {                        string tempLine = string.Empty;                        using (StreamWriter writer =new StreamWriter(fileName))                        {                            while (reader.Read())                            {                                tempLine = reader["AreaId"] + "," +                                           reader["AreaName"] + "," +                                           reader["AreaPId"];                                writer.WriteLine(tempLine);//写入文本文件                            }                        }                                          }                }            }        }    }}
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