Populating listbox control and textbox control from database

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.OleDb;
namespace Autopopulate_a_textbox
{
    public partial class Form2 : Form
    {
        string ConnectionString = System.Configuration.ConfigurationSettings.AppSettings["dsn"];
        OleDbCommand com;
        string str;

        public Form2()
        {
            InitializeComponent();
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            comboBox1.Text="Choose ID";
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from customer";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                comboBox1.Items.Add(reader["id"].ToString());
            }
            con.Close();
            reader.Close();
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            listBox1.Items.Clear();
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from customer where id='" + comboBox1.Text.Trim() + "'";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                listBox1.Items.Add(reader["address"].ToString());
                textBox1.Text = reader["vatno"].ToString();
            }
            con.Close();
            reader.Close();
        }
    }
}