[SOLVED] Excel-dna Add-in opens Excel in the background after closing Excel

Excel-dna Add-in opens Excel in the background after closing Excel

I wrote an Add-In using Excel-DNA. Testing it in different Excel Versions (2016,365 on different PC with Win 10) with Excel-Add-Ins getting the .xll Works fine - but when Excel is closed, a hidden Excel Instance is created/kept? Which one has to be killed by task manager. (Or reopening Excel will not open with add-ins)

When I unload the add-in, Excel closes completely.

A strange thing is, that the Add-In is shown in the Excel Add-Ins also in the com Add-Ins

What could be the reason for that (mis)behaviour and how can I solve this.

Thankful for every advice

P.S. ExcelDna 1.6.0, .Net Framework 4.7.2, vb.net

Solution

What happens is that the Excel process is not terminating because there are outstanding COM references. From your Excel-DNA add-in, this can happen easily if you access the Excel COM object model from a thread that is not the main thread (ManagedThreadId==1). From an Excel-DNA add-in you should only ever access the COM object model from the main thread.

Some examples that cause problems would be:

Using the COM object model inside an async function's 'inside' part.
Having a timer callback from a threadpool thread where you do some work with the COM object model.
Having some async work or a Task that calls the object model when it completes.

In some of these cases you can switch to the main thread in a context where it is safe to call the COM object model with a call to the Excel-DNA helper method ExcelAsyncUtil.QueueAsMacro. This can be called from any thread, and the code will execute as soon as Excel is available.