Why do instances of Excel not close when opened via interop, even though Word apps close just fine?

Question

As far as I can see, this is not a duplicate question, as the question here is about why the accepted answers of seemingly duplicate questions, do not solve my issue in what appears to be the same circumstances.

For days, I have been struggling with my application failing to end instances of Excel, which have been opened using interops.

My application opens and closes Word applications just fine (ie the process disappears from task manager), but not Excel apps. Excel always remains open in the task manager. After much, much searching - this seems to be the definite guide on how to close Excel instances. However , if I copy and paste that exact example code - and run it as a console app, in either .NET Framework, or .NET 6 - Excel still fails to close, exactly as per the behaviour of my application.

Presumably, this must be an issue with the latest/newer versions of Word?

Completely and utterly lost on this one...

Code from the above link that leaves Excel open:

using System;
using System.Runtime.InteropServices;
using Microsoft.Office.Interop.Excel;

namespace TestCsCom
{
    class Program
    {
        static void Main(string[] args)
        {
            // NOTE: Don't call Excel objects in here... 
            //       Debugger would keep alive until end, preventing GC cleanup

            // Call a separate function that talks to Excel
            DoTheWork();

            // Now let the GC clean up (repeat, until no more)
            do
            {
                GC.Collect();
                GC.WaitForPendingFinalizers();
            }
            while (Marshal.AreComObjectsAvailableForCleanup());
        }

        static void DoTheWork()
        {
            Application app = new Application();
            Workbook book = app.Workbooks.Add();
            Worksheet worksheet = book.Worksheets["Sheet1"];
            app.Visible = true;
            for (int i = 1; i <= 10; i++) {
                worksheet.Cells.Range["A" + i].Value = "Hello";
            }
            book.Save();
            book.Close();
            app.Quit();

            // NOTE: No calls the Marshal.ReleaseComObject() are ever needed
        }
    }
}

Answer 1

It does seem a bit weird with the current version of Excel. It depends on how the console program is run. If I double-click in Windows Explorer, it closes perfectly like it used to. But if I run from a console window, then Excel stays open until the console window closes. Similarly when I run from the debugger, Excel only closes when the Debug window is closed and not when the driving process is terminated. It would be good if you can confirm this too. I suspect Excel has added some defensive code to not close in specific situations - maybe watching for a 'parent' window to close, or something similar.

Aaaaand then. . . . just as I try again after posting this, Excel starts closing beautifully every time, however I run it.

Answer 2

OK, so I've stumbled across the solution to this. The solution is really rather an unrelated issue.

My instance of Excel had the Analysis Toolpak enabled (spelling mistake included,). from some work I was doing a few months back. The add-in is found at File --> Options --> Add-ins --> Analysis ToolPak .

I've disabled the add-in, and now Excel apps open and close just fine, using only explicit garbage collection, as expected.

I should add - the behaviour is still different to Word. Closed Word files kill Word almost instantly on closure of the document, even before the explicit GC. With Excel, I still have the close the form where the Workbook was opened from, which then calls the explicit GC, and sometimes it takes a few seconds, sometimes 30 seconds, sometimes it takes a minute - but having disabled the add-in, Excel does eventually get stopped.