Close Exel Task in Taskmanager

Question

I got a problem with accessing an excel file and closing it afterwards completely. I read a lot of forum entrys and articles about it and my code is a lot better than at the start. So currently i I am activating line after line and I am checking if the closing is still possible. The codeline which is causing trouble is marked below. With this active, the excel task is still present in my task manager. I think it has something to do with the reading of the cell value, but I dont understand whats the problem. The reading has no problem, only the task will be still open in my taskmanager.

One more thing: The Function is called inside a Backgroundworker, if this is important.

Thank you for all your help and sorry for my formulation! If something is not clear, please ask and I will try to explain it better

 public string CallExcel(string releaseDescription = "", string releaseCntNr = "")
    {
        Excel.Application ExcelApp = null;
        Excel.Workbooks books = null;
        Excel.Workbook workbook = null;
        Excel.Sheets sheets = null;
        Excel.Worksheet worksheet = null;
        Excel.Range range = null;
        Excel.Range testrange = null;
        Excel.Range rows = null;
        Excel.Range cells = null;

        try
        {
            ExcelApp = new Excel.Application();
            ExcelApp.DisplayAlerts = false;
            books = ExcelApp.Workbooks;
            workbook = books.Open("C:\\temp\\Tooltest\\BB-No_for_ACC_STIL_routines.xlsx");
            sheets = workbook.Sheets;
            worksheet = sheets[Math.Min(0x00000001, sheets.Count)];
            range = worksheet.UsedRange;
            
            rows = range.Rows;
            cells = range.Cells;

            //PROBLEMATIC LINE:
            var testvar = cells[0x0000006c, 0x00000002].Value;

            //for (uint iRow = 0x0000006a; iRow <= rows.Count; iRow++)
            //{ blabla}                
        }
        catch (Exception)
        {
            throw;
        }
        finally
        {
            //Release in opposite order
            if (cells != null) Marshal.ReleaseComObject(cells);  //Not sure if neccessary
            if (rows != null) Marshal.ReleaseComObject(rows);    //Not sure if neccessary
            if (testrange != null) Marshal.ReleaseComObject(testrange);
            if (range != null) Marshal.ReleaseComObject(range);
            if (worksheet != null) Marshal.ReleaseComObject(worksheet);
            if (sheets != null) Marshal.ReleaseComObject(sheets);
            if (workbook != null)
            {
                workbook.Close(true, Type.Missing, Type.Missing);
                Marshal.ReleaseComObject(workbook);
            }
            if (books != null) Marshal.ReleaseComObject(books);
            if (ExcelApp != null)
            {
                ExcelApp.Quit();
                Marshal.ReleaseComObject(ExcelApp);
            }
        }
        // Cleanup
        GC.Collect();
        GC.WaitForPendingFinalizers();

    return "something";
    }