<?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 SqlDataAdapterusing System.Data.SqlClient;//SqlConnection Command DataReadernamespace 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</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  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.去執行sql語句，讀取數據庫  //3.sqlDataReader,把讀取到的數據填充到內存表中  adapter.Fill(dt); } } return dt; } /// <summary> /// 執行查詢，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> /// 執行增刪改，返回受影響的行數 /// </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 ，去數據庫查找省的數據，給cboPro DataTable dt = SqlHelper.ExecuteDataTable("select * from promary"); //將返回的DataTable作為cboPro的數據源 //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());//獲得關聯的數據 //string sql = "select * from city where proID="+cboPro.SelectedValue.ToString(); //帶參數的sql語句 string sql = "select * from city where proID=@proID"; //準備一個sql參數 SqlParameter p = new SqlParameter("@proID", cboPro.SelectedValue.ToString()); //設置cboCity要顯示的數據 cboCity.DisplayMember = "cityName"; //根據sql語句查詢到的數據集 cboCity.DataSource = SqlHelper.ExecuteDataTable(sql, p); } }