On and off I’ve been writing an Office add-in for Outlook. We’ve reached the point of testing, but when we installed it on a clean machine, it wouldn’t run. There was an error message complaining about not having the Extensibility.dll installed. “Odd” we thought. We’d been careful to install the Outlook PIAs (Primary Interop Assemblies), and no-one had heard of this assembly before.
Well, it appears that this assembly is installed with Visual Studio, but it isn’t installed with the PIAs. Nice. Fortunately Gunnar Peipman has an extremely timely post on this – I’d been looking at the Extensibility.dll in the GAC, but apparently there is one under Common Files too. I’m not sure where is has to go though – the GAC, the application folder, or into the common files directory. I’ll report back when I figure this out…
What I’d really like to know, though, is why this file isn’t part of the PIAs? I mean, what are you going to do with them if you aren’t writing an add-in?
Andy Burns' Blog 
[…] was surprising. Extensibility.dll isn’t one of mine – it’s a Microsoft one that should’ve been in the Primary Interop Assemblies for Office. Anyway, it’s not one of mine, it hadn’t changed, and it was signed correctly. So what […]