c#省市联动（sqlHelper的应用）

sqlHelper：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Configuration;
using System.Data.SqlClient;

namespace WindowsFormsApplication1
{
    public class sqlHelper
    {
        private static readonly string str = ConfigurationManager.ConnectionStrings["conStr"].ConnectionString;
        //增删改
        public static int ExecuteNonquery(string sql, params SqlParameter[] ps)
        {
            using (SqlConnection con = new SqlConnection(str))
            {
                using (SqlCommand cmd = new SqlCommand(sql, con))
                {
                    con.Open();
                    if (ps != null)
                    {
                        cmd.Parameters.AddRange(ps);
                    }
                    return cmd.ExecuteNonQuery();
                }
            }

        }
        //首行首列
        public static object ExecuteScalar(string sql, params SqlParameter[] ps)
        {
            using (SqlConnection con = new SqlConnection(str))
            {
                using (SqlCommand cmd = new SqlCommand(sql, con))
                {
                    con.Open();
                    if (ps != null)
                    {
                        cmd.Parameters.AddRange(ps);
                    }
                    return cmd.ExecuteScalar();
                }
            }
        }
        //查询
        public static SqlDataReader ExecuteReader(string sql, params SqlParameter[] ps)
        {
            SqlConnection con = new SqlConnection(str);
            using (SqlCommand cmd = new SqlCommand(sql, con))
            {
                if (ps != null)
                {
                    cmd.Parameters.AddRange(ps);
                }
                try
                {
                    con.Open();
                    return cmd.ExecuteReader(System.Data.CommandBehavior.CloseConnection);
                }
                catch (Exception ex)
                {
                    con.Close();
                    con.Dispose();
                    throw ex;
                }
            }
        }
    }
}

Form1设计：

Form1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.Threading.Tasks;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            LoadShengById2(0);
        }

        private void LoadShengById2(int p)
        {
            string sql = "select id1, name from city where id2=" + p;
            List<City> list = new List<City>();
            list.Add(new City() { Id1 = -1,Name = "请选择"});//加一个“请选择”，如果不加，点北京不出城市，为什么？
            using (SqlDataReader reader = sqlHelper.ExecuteReader(sql))
            {
                if (reader.HasRows)
                {
                    while (reader.Read())
                    {
                        City c = new City();
                        c.Id1 = Convert.ToInt32(reader["id1"]);
                        c.Name = reader["name"].ToString();
                        //c.Id2 = Convert.ToInt32(reader["id2"]);
                        list.Add(c);
                    }

                }
            }
            //下面的放在using外面
            cobSheng.DataSource = list;
            cobSheng.DisplayMember = "Name";//显示的是哪个名字的信息
            cobSheng.ValueMember = "Id1";//实际保存的信息

        }

        private void cobSheng_SelectedIndexChanged(object sender, EventArgs e)
        {
            //一定要加判断，有选中才运行！
            if (cobSheng.SelectedIndex != 0)
            {
                //MessageBox.Show(cobSheng.SelectedValue.ToString());
                int id = Convert.ToInt32(cobSheng.SelectedValue);
                string sql = "select * from city where id2=" + id;
                List<City> list = new List<City>();
                using (SqlDataReader reader = sqlHelper.ExecuteReader(sql))
                {
                    if (reader.HasRows)
                    {
                        while (reader.Read())
                        {
                            City c = new City();
                            c.Id1 = Convert.ToInt32(reader["id1"]);
                            c.Name = reader["name"].ToString();
                            c.Id2 = Convert.ToInt32(reader["id2"]);
                            list.Add(c);
                        }
                        cobShi.DataSource = list;
                        cobShi.DisplayMember = "name";
                    }
                }
            }


        }
    }
}

City类：

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

namespace WindowsFormsApplication1
{
    public class City
    {
        private int _id1;

        public int Id1
        {
            get { return _id1; }
            set { _id1 = value; }
        }
        private string _name;

        public string Name
        {
            get { return _name; }
            set { _name = value; }
        }
        private int _id2;

        public int Id2
        {
            get { return _id2; }
            set { _id2 = value; }
        }
    }
}

App.config:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
<connectionStrings>
  <add connectionString="Data Source =.; Initial Catalog = mysql; Integrated Security = True;" name="conStr" />
</connectionStrings>
</configuration>

 数据库：