为什么列表框2仍然为空

我找不到问题出在哪里，当我把while循环作为注释时，它是有效的，在while循环之后，我们不能从sqldatareader中读取更多？listbox2仍然为空！

public partial class exTest : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string connectionString = WebConfigurationManager.ConnectionStrings["Northwind"].ConnectionString;
        SqlConnection con = new SqlConnection(connectionString);
        string sql = "select employeeid,firstname,lastname from employees";
        SqlCommand cmd = new SqlCommand(sql, con);
        con.Open();
        SqlDataReader reader = cmd.ExecuteReader();
        string item;
        ListBox1.Items.Clear();
        while (reader.Read())
        {
            item =reader.GetString(1) + reader.GetString(2);
            ListBox1.Items.Add(item);
        }
        ListBox2.DataSource = reader;
        ListBox2.DataValueField = "employeeId";
        ListBox2.DataTextField = "firstname";
        ListBox2.DataBind();
        reader.Close();
        con.Close();
    }
}