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

using System.Configuration;
using System.Data;//DatSet..Table SqlDataAdapter
using System.Data.SqlClient;//SqlConnection Command DataReader
namespace Common
{
 public class SqlHelper
 {
 //连接字符串
 //1、添加引用 2、导入命名空间 为了使用ConfigurationManager
 private static string conStr = ConfigurationManager.ConnectionStrings["conStr"].ConnectionString;

 //增删改查
 //查找数据 ExecuteScalar()返回首行首列 ExecuteReader() DataTable


 /// <summary>
 /// 返回DataTable
 /// </summary>
 /// <param name="sql">所用的sql语句</param>
 /// <param name="param">可变，可以传参也可以不传参数</param>
 /// <returns></returns>
 public static DataTable ExecuteDataTable(string sql, params SqlParameter[] param)
 {
 DataTable dt = new DataTable();
 using (SqlConnection con = new SqlConnection(conStr))
 {
 using (SqlDataAdapter adapter = new SqlDataAdapter(sql, con))
 { 
  //添加参数
  adapter.SelectCommand.Parameters.AddRange(param);
  //1.打开链接，如果连接没有打开，则它给你打开；如果打开，就算了
  //2.去执行sql语句，读取数据库
  //3.sqlDataReader,把读取到的数据填充到内存表中
  adapter.Fill(dt);
 }
 }
 return dt;
 }

 /// <summary>
 /// 执行查询，返回首行首列
 /// </summary>
 /// <param name="sql"></param>
 /// <param name="param"></param>
 /// <returns></returns>
 public static object ExecuteScalar(string sql, params SqlParameter[] param)
 {
 object o = null;
 using (SqlConnection con = new SqlConnection(conStr))
 {
 using (SqlCommand cmd = new SqlCommand(sql, con))
 {
  cmd.Parameters.AddRange(param);
  con.Open();

  o = cmd.ExecuteScalar();
 }
 }
 return o;
 }


 /// <summary>
 /// 执行查询，返回SqlDataReader对象
 /// </summary>
 /// <param name="sql"></param>
 /// <param name="param"></param>
 /// <returns></returns>
 public static SqlDataReader ExecuteReader(string sql, params SqlParameter[] param)
 {
 SqlDataReader reader = null;
 using (SqlConnection con = new SqlConnection(conStr))
 {
 using (SqlCommand cmd = new SqlCommand(sql, con))
 {
  cmd.Parameters.AddRange(param);
  con.Open();

  reader = cmd.ExecuteReader();
 }
 }
 return reader;
 }

 /// <summary>
 /// 执行增删改，返回受影响的行数
 /// </summary>
 /// <param name="sql"></param>
 /// <param name="param"></param>
 /// <returns></returns>
 public static int ExecuteNonQuery(string sql, params SqlParameter[] param)
 {
 int n = -1;
 using (SqlConnection con = new SqlConnection(conStr))
 {
 using (SqlCommand cmd = new SqlCommand(sql, con))
 {
  cmd.Parameters.AddRange(param);
  con.Open();
  n = cmd.ExecuteNonQuery();
 }
 }
 return n;
 }

 }
}

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

 private void Form1_Load(object sender, EventArgs e)
 {
 //在应用程序加载的时候 ，去数据库查找省的数据，给cboPro
 DataTable dt = SqlHelper.ExecuteDataTable("select * from promary");
 //将返回的DataTable作为cboPro的数据源
 //让cboPro显示proName这个字段的值，一般是显示给客户看的
 cboPro.DisplayMember = "proName";
 //让valueMemberID，绑定的是对应的值，绑定处理程序标识 给程序员看的。
 cboPro.ValueMember = "proID";
 cboPro.DataSource = dt;
 }

 private void cboPro_SelectedIndexChanged(object sender, EventArgs e)
 {
 //之前的写法
 // MessageBox.Show(cboPro.Text);//获得在cbo中选择文本
 //MessageBox.Show(cboPro.SelectedValue.ToString());//获得关联的数据

 //string sql = "select * from city where proID="+cboPro.SelectedValue.ToString();
 //带参数的sql语句
 string sql = "select * from city where proID=@proID";
 //准备一个sql参数
 SqlParameter p = new SqlParameter("@proID", cboPro.SelectedValue.ToString());
 //设置cboCity要显示的数据
 cboCity.DisplayMember = "cityName";
 //根据sql语句查询到的数据集
 cboCity.DataSource = SqlHelper.ExecuteDataTable(sql, p);
 }
 }