帮忙看一下这道题的代码 为什么无法查询表的内容

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
namespace HomeWork9
{
public partial class Form2 : Form
{
public Form2()
{
InitializeComponent();
}
/// <summary>
/// 加载事件
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void Form2_Load(object sender, EventArgs e)
{
//定义sql语句
string sql = "select cardID,cardNumber from CardInfo";
//定义连接字符串
string connStr = "server=.;database=CardDB;uid=sa;pwd=123456";
//创建连接对象
SqlConnection conn = new SqlConnection(connStr);
//打开
conn.Open();
//创建数据适配器
SqlDataAdapter sda = new SqlDataAdapter(sql, conn);
//创建数据集
DataSet ds = new DataSet();
//将sda取到的数据填充到ds中
sda.Fill(ds);
//关闭数据库
conn.Close();
//将数据绑定下拉列表框
this.comboBox1.DataSource = ds.Tables[0];
//指定要显示的列
this.comboBox1.DisplayMember = "cardNumber";
//指定隐藏值
this.comboBox1.ValueMember = "cardid";
}
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
//得到选中的项
string id = this.comboBox1.SelectedValue.ToString();
//定义sql语句
string sql = string.Format("select * from cardInfo where cardNumber='{0}'", id);
string connStr = "server=.;database=CardDB;uid=sa;pwd=123456";
SqlConnection conn = new SqlConnection(connStr);
conn.Open();
SqlDataAdapter sda = new SqlDataAdapter(sql, conn);
DataSet ds = new DataSet();
sda.Fill(ds);
conn.Close();
//将dataset中的数据显示到对应的文本框
this.textBox1.Text = ds.Tables[0].ToString();
this.textBox2.Text = ds.Tables[0].ToString();
this.textBox3.Text = ds.Tables[0].ToString();
}
}
}