Automating Excel and Add-ins Don’t load


There is a common problem I see when automating Excel from an external .NET application:

You will launch Excel, open a workbook, perform work, and then try to close Excel and find that it remains in memory. You might even have code in your solution to try to re-use existing instance of Excel and find that it uses one of these “zombie” instances that do not have any of the add-ins loaded… and maybe an add-in your code relies on.

The issue is that Excel is not able to close because  the primary thread of your application is holding on to a COM reference. Even adding Marshal.ComReleaseObject() and GC.Collect() like this probably will not improve the situation either:

From my experience, and from what I have gathered is that until the thread is stopped, the COM reference will be locked. Not sure where the issue lies (EXCEL, COM, or .NET), but I have found a workaround to this. What you have to do is terminate your thread. But because you do not want to terminate your application, you need to create a NEW thread. So the following pattern is what I have found works:

 

 

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