DropDownList QueryString C#

Hello.


I am trying make the dropdownlist, with AutoPostBack="true", to narrow the results in a search.
First problem is that the page doesen't remenber the css link i made neither the contents of the dropdownlist. here is the code for populating the list:
if(!IsPostBack)
{
       CategorieSupraDropDownList.DataSource = CatalogAccess.GetCategorieSupra();
       CategorieSupraDropDownList.DataValueField = "CategorieSupraID";
       CategorieSupraDropDownList.DataTextField = "CategorieSupraNume";
       CategorieSupraDropDownList.DataBind();
       CategorieSupraDropDownList.Items.Insert(0, new ListItem("--- Selecteaza categorie ---", ""));

 
     HtmlLink cssLink = new HtmlLink();

     cssLink.Href = "~/Css/Branduri.css";

     cssLink.Attributes.Add("rel&q ...