Passing multiple value in querystring and ignoring 0 index value of dropdownlist

 protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
    {
        Response.Redirect(string.Format("Collage_result.aspx?cat={0}&loc={1}", (dropcategory.SelectedIndex == 0 ? (object)string.Empty : dropcategory.SelectedItem), dropcity.SelectedItem));
    }