Excel process remains open after interop; traditional method not working

Question

I'm running into an issue with some code I'm debugging. Excel interop is used to extract some values from a workbook; however, Excel remains open after the program has exited. I've tried the traditional solution, but it still keeps a reference to Excel open on all machines where the code is run

private void TestExcel()
    {
        Excel.Application excel = new Excel.Application();
        Excel.Workbooks books = excel.Workbooks;
        Excel.Workbook book = books.Open("C:\\test.xlsm");

        book.Close();
        books.Close();
        excel.Quit();

        Marshal.ReleaseComObject(book);
        Marshal.ReleaseComObject(books);
        Marshal.ReleaseComObject(excel);
    }

Even this simple piece of code keeps the process running with multiple files (xlsm, xlsx, xls). Right now we have a workaround in place to kill the Excel processes we've opened, but I'd much rather get this working for my own sanity.

I should add that I have it narrowed down to the Workbook variable. If I remove the call to books.Open() and all references to book then it closes successfully.

Answer 1

This has worked successfully for me:

        xlApp.Quit();

        //release all memory - stop EXCEL.exe from hanging around.
        if (xlWorkBook != null) { Marshal.ReleaseComObject(xlWorkBook); } //release each workbook like this
        if (xlWorkSheet != null) { Marshal.ReleaseComObject(xlWorkSheet); } //release each worksheet like this
        if (xlApp != null) { Marshal.ReleaseComObject(xlApp); } //release the Excel application
        xlWorkBook = null; //set each memory reference to null.
        xlWorkSheet = null;
        xlApp = null;
        GC.Collect();