C# Proper Way to Close Multiple Excel Objects With

I have a C# console program. When it runs, it instantiates some Excel objects such as: (openExcelO).

When the program is done running, I have a method to close down Excel (closeExcel) and that is supposed to properly clean up the Excel process. In task manager I can see the remnants of Excel that remain. The shut down routines were cobbled together from internet examples:

    private void openExcelO (string dir) {

        try {
            xlApp = new Excel.Application();
            xlWorkBook = xlApp.Workbooks.Open(dir + "PortfolioOptimization5.xlsm",
                0, false, 5, "", "", true, 
                Microsoft.Office.Interop.Excel.XlPlatform.xlWindows, 
                "\t", false, false, 0, true, 1, 0);
            xlMarkowitz = (Excel.Worksheet)xlWorkBook.Sheets["Markowitz"];
            xlWeights = (Excel.Worksheet)xlWorkBook.Sheets["Weights"];

        } catch (Exception ex) {
            ReportError("PortfolioOptimization", "openExcel", ex.ToString());
        }

    }


    private void closeExcel () {
        object misValue = System.Reflection.Missing.Value;
        try {

            if (xlWeights != null) releaseObject(xlWeights);
            if (xlMarkowitz != null) releaseObject(xlMarkowitz);

            xlWorkBook.Close(false, misValue, misValue);
            if (xlApp != null) xlApp.Quit();

            if (xlWorkBook != null) releaseObject(xlWorkBook);
            if (xlApp != null) releaseObject(xlApp);

        } catch (Exception ex) {
            ReportError("PortfolioOptimization", "closeExcel", ex.ToString());
        }
    }


    private void releaseObject (object obj) {
        try {
            System.Runtime.InteropServices.Marshal.ReleaseComObject(obj);
            obj = null;
        } catch (Exception ex) {
            obj = null;
            ReportError("PortfolioOptimization", "releaseObject", "Unable to release object " + ex.ToString());
        } finally {
            GC.Collect();
        }
    }

How can I change the shutdown routine to properly shut down and clean up the Excel objects so there are no lingering Excel processes in the Task Manager?

Thanks!

It looks like you're not releasing your objects in the correct order - xlApp.Quit probably can't exit cleanly because you still hold a reference to the workbook. You need to release the workbook before you invoke Quit:

xlWorkBook.Close(false, misValue, misValue);
releaseObject(xlWorkBook);

xlApp.Quit();
releaseObject(xlApp);

Credit must go to this StackOverflow answer: there are a couple of other implicit references that you're not closing, i.e., in the following lines:

xlWorkBook = xlApp.Workbooks.Open(...)
xlWorkBook.Sheets["Markowitz"];

The "Workbooks" object isn't being released, and neither is the "Sheets" object. You need to release these references too, i.e.:

var books = xlApp.Workbooks;
books.Open(...);
releaseObject(books);

var sheets = xlWorkBook.Sheets;
xlMarkowitz = (Excel.Worksheet)sheets["Markowitz"];
xlWeights = (Excel.Worksheet)sheets["Weights"];
releaseObject(sheets);

Great advice from Bruce Barker: "Never use two dots with COM objects"!