<?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
 {
 //連接字符串
 //1、添加引用 2、導入命名空間 為了使用ConfigurationManager
 private static string conStr = ConfigurationManager.ConnectionStrings["conStr"].ConnectionString;

 //增刪改查
 //查找數(shù)據 ExecuteScalar()返回首行首列 ExecuteReader() DataTable


 /// <summary>
 /// 返回DataTable
 /// </summary>
 /// <param name="sql">所用的sql語句</param>
 /// <param name="param">可變，可以傳參也可以不傳參數(shù)</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))
 { 
  //添加參數(shù)
  adapter.SelectCommand.Parameters.AddRange(param);
  //1.打開鏈接，如果連接沒有打開，則它給你打開；如果打開，就算了
  //2.去執(zhí)行sql語句，讀取數(shù)據庫
  //3.sqlDataReader,把讀取到的數(shù)據填充到內存表中
  adapter.Fill(dt);
 }
 }
 return dt;
 }

 /// <summary>
 /// 執(zhí)行查詢，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>
 /// 執(zhí)行查詢，返回SqlDataReader對象
 /// </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>
 /// 執(zhí)行增刪改，返回受影響的行數(shù)
 /// </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)
 {
 //在應用程序加載的時候 ，去數(shù)據庫查找省的數(shù)據，給cboPro
 DataTable dt = SqlHelper.ExecuteDataTable("select * from promary");
 //將返回的DataTable作為cboPro的數(shù)據源
 //讓cboPro顯示proName這個字段的值，一般是顯示給客戶看的
 cboPro.DisplayMember = "proName";
 //讓valueMemberID，綁定的是對應的值，綁定處理程序標識 給程序員看的。
 cboPro.ValueMember = "proID";
 cboPro.DataSource = dt;
 }

 private void cboPro_SelectedIndexChanged(object sender, EventArgs e)
 {
 //之前的寫法
 // MessageBox.Show(cboPro.Text);//獲得在cbo中選擇文本
 //MessageBox.Show(cboPro.SelectedValue.ToString());//獲得關聯(lián)的數(shù)據

 //string sql = "select * from city where proID="+cboPro.SelectedValue.ToString();
 //帶參數(shù)的sql語句
 string sql = "select * from city where proID=@proID";
 //準備一個sql參數(shù)
 SqlParameter p = new SqlParameter("@proID", cboPro.SelectedValue.ToString());
 //設置cboCity要顯示的數(shù)據
 cboCity.DisplayMember = "cityName";
 //根據sql語句查詢到的數(shù)據集
 cboCity.DataSource = SqlHelper.ExecuteDataTable(sql, p);
 }
 }