how can get selectedindex in dropdown list when i use dropdownlist in html code and this is into the datalist this means i havenot use findcontrol in code behine beacuse this object not initialize

<asp:DropDownList ID="Dropdownlist1" Runat="server" DataSource='<%#FillGroup()%>' DataTextField="cat_name" DataValueField="Cat_id" >
public System.Data.DataSet FillGroup()//--new for forum App
        {
            System.Data.DataSet DS = new System.Data.DataSet();

            sqlCmd.CommandText =" SELECT Cat_id,cat_name FROM Category where c_statuse=1 and C_public=0 ";
            DS.Clear();
            sqlAdp.Fill(DS);
            System.Data.DataRow dr = DS.Tables[0].NewRow();
            dr["Cat_id"] = "-1";
            dr["cat_name"] = "ÇäÊÎÇÈ ÇÒ áíÓÊ...";
            DS.Tables[0].Rows.InsertAt(dr, 0);
            return DS;
        }
        public string FillDropDown(object ,System.EventArgs e )
        {
            DataListItem anItem in datalist (is this correct

            DropDownList dropdown =((DropDownList)anItem.FindControl("Dropdownlist1" ));

             dropdown.SelectedItem.Value=(string)Session["result1"].ToString();;
            return dropdown.SelectedItem.Value;
        }
thank's for your help