.NET Interoperability with COM Office Apps, a Hidden Gotcha

Filed under .NET, ActiveX, Office, Troubleshooting, VB Feng Shui

image I’ve been working with .NET and Office interoperability for some time now, and, other than a few minor hiccups here and there, things have generally been very smooth.

But a colleague recently ran into a problem that required a good deal of hunting to resolve. Long story short, even though .NET interoperability with Office objects is virtually foolproof, releasing your references has some hidden dangers lurking amongst the weeds.

As a general rule, it’s safe to let the .NET garbage collector (GC) do it’s thing while your application is running.

BUT, when you’re app shuts down, you need to take a few extra steps to make sure everything’s cleaned up before quitting.

The details are laid on in this article on MSDN. It’s from 2005, but, from what I can tell, it’s just as applicable now as it was then.

Essentially, the trick boils down to this:

When you’re preparing to shut your application down, you need to make SURE you’ve released all your references to any Office objects. This means setting all your various references to Nothing, or, more typically, letting the GC do its thing.

Unfortunately, the GC doesn’t necessarily do its thing during a shutdown, so you have to force the issue. And that requires code essentially like this:

WordApp = Nothing 

The article noted above doesn’t really go into why it’s necessary to call Collect and WaitForPendingFinalizers twice, but it does appear to be required. I’m guessing it’s due to the way the Finalizer sweeps object references; it could be possible for it to release objects in a certain order, causing it to skip some objects. But that’s just a guess.

And as a final note, if your .NET code uses any COM interoperability, it might be a good idea to do some final cleanup along these lines as well.

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