Data not binding to dropdown list

private void load_uname()
    {
username.Items.Clear();

        SqlConnection con = new SqlConnection(constring);
        con.Open();
        string sql = "select [uname] from user_registration where usergroup ='" + usertype.SelectedValue.ToString() + "' ";
        SqlCommand cmd = new SqlCommand(sql, con);
        SqlDataReader dr;
        dr = cmd.ExecuteReader();
        username.Items.Add("<------Select---->");
        while (dr.Read())
        {

            username.Items.Add(new ListItem(dr[0].ToString()));
        }
    }