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();
         }

         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 = -,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 != )
             {
                 //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：