How to bind data into dropdownlist

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
public partial class Default3 : System.Web.UI.Page
{
    string strConnString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
    string str;
    SqlCommand com;
    protected void Page_Load(object sender, EventArgs e)
    {
        DropDownList1.AutoPostBack = true;
        SqlConnection con = new SqlConnection(strConnString);

        if (!IsPostBack)
        {
            DropDownList1.Items.Add("Choose userid");
            con.Open();
            str = "select * from Contacts";
            com = new SqlCommand(str, con);
            SqlDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                DropDownList1.Items.Add(reader["userid"].ToString());
            }
            reader.Close();
            con.Close();
        }
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(strConnString);
        con.Open();
        str = "select * from Contacts where userid='" + DropDownList1.SelectedItem.Text + "'";
        com = new SqlCommand(str, con);
        SqlDataReader reader = com.ExecuteReader();
        while (reader.Read())
        {
            TextBox1.Text=reader["Fname"].ToString();
            TextBox2.Text = reader["Lname"].ToString(); 
        }
        reader.Close();
        con.Close();
    }
}