How to clean excel interop object after closing excel

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C#

C#4.0

When I am running following code Excel object remains in task manager.

Excel.Application appExl = new Excel.Application();
Excel.Workbook workbook = null;
Excel.Worksheet nwSheet = null;
Excel.Range last = null, range = null;

//Open Excel

workbook = appExl.Workbooks.Open(@txtBrowse.Text, Missing.Value, false, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value);

nwSheet = (Excel.Worksheet)workbook.Sheets.get_Item(1);

last = nwSheet.Cells.SpecialCells(Excel.XlCellType.xlCellTypeLastCell, Type.Missing);
range = nwSheet.get_Range("A1", last);

int lastUsedRow = last.Row;
nwSheet.Cells[1, 1].Value2 = "test";
workbook.Save();

if (last != null)
                    Marshal.FinalReleaseComObject(last);
                last = null;
                if (range != null)
                    Marshal.FinalReleaseComObject(range);
                range = null;
                if (nwSheet != null)
                    Marshal.FinalReleaseComObject(nwSheet);
                nwSheet = null;
                if (workbook != null)
                {
                    workbook.Close(true, Missing.Value, Missing.Value);
                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(workbook);
                }
                workbook = null;
                if (appExl != null)
                {
                    ((Excel._Application)appExl).Quit();
                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(appExl);
                }
                appExl = null;

GC.Collect();


I have tried this one also

workbook.Close();
Marshal.ReleaseComObject(workbook);
appExl.Quit();
Marshal.ReleaseComObject(appExl);
workbook=null;
appExl=null;
GC.Collect();