Åbne eksisterende excel ark

Langt om længe fandt jeg en løsning:

string folderadress = "~/Excel/Hej.xls";
        folderadress = HttpContext.Current.Server.MapPath(folderadress);
        string storefolderadress = "~/Excel";
        storefolderadress = HttpContext.Current.Server.MapPath(storefolderadress);
        Microsoft.Office.Interop.Excel.Application WRExcel = new Microsoft.Office.Interop.Excel.Application();
        Microsoft.Office.Interop.Excel.Workbooks WRwbs = null;
        //Microsoft.Office.Interop.Excel.Workbook WRwb = new Microsoft.Office.Interop.Excel.Workbook();
        Microsoft.Office.Interop.Excel.Workbook WRwb = WRExcel.Workbooks.Add(Excel.XlWBATemplate.xlWBATWorksheet);
        Microsoft.Office.Interop.Excel._Worksheet WRws = null;
        object objOpt = System.Reflection.Missing.Value;

        WRExcel.Visible = false;
        WRwbs = WRExcel.Workbooks;

        WRwb = WRwbs.Open(folderadress, objOpt, objOpt, objOpt, objOpt, objOpt, objOpt, objOpt, objOpt,
            objOpt, objOpt, objOpt, objOpt, objOpt, objOpt);
        Microsoft.Office.Interop.Excel.Sheets WRss = null;
        WRss = WRwb.Sheets;
        WRws = (Microsoft.Office.Interop.Excel.Worksheet)WRss.get_Item(1);
        WRws.Cells[1, 1] = "Zone";
        WRwb.SaveAs(storefolderadress + "\\" + "Excel1.xls", objOpt, objOpt, objOpt, objOpt, objOpt, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange, objOpt, objOpt, objOpt, objOpt, objOpt);
        WRwbs.Close();

problemet lå i:
Microsoft.Office.Interop.Excel.Workbook WRwb = new Microsoft.Office.Interop.Excel.Workbook();

skulle byttes ud med:
Microsoft.Office.Interop.Excel.Workbook WRwb = WRExcel.Workbooks.Add(Excel.XlWBATemplate.xlWBATWorksheet);