Avoid the GAC

Thursday, March 11th, 2004

The .NET Global Assembly Cache (GAC) is a misunderstood and misused beast. For all intents and purposes, it provides what COM and windows\system32 do, i.e. a machine-wide place to drop shared DLLs. Of course, the problems with sharing DLLs in a machine-wide spot is that it leads to a set of well-known problems collectively called "DLL Hell." There are many problems, but the biggest is that when a shared DLL is updated, you're really updating an unknown set of applications. If the set of applications is unknown, how can you possible test them before making this change? And if you can't test them, you're likely to break them. What this boils down to is that any of the shared spots for updates, whether it's a COM CLSID, windows\system32 or the GAC, are dangerous and should be avoided. And this is why the preferred .NET deployment scenario is "xcopy deployment," i.e. having your own private copy of each DLL that you test and deploy with the rest of your application.

"Aha!" you say. "The GAC supports multiple version of an assembly! When a foo.dll is updated to v1.1, v1.0 sits right along side of it so that your app *doesn't* break!" Of course, that's absolutely true. But if that's the case, why do you care? I mean, if there's a new assembly available but your app isn't picking it up, what difference does it make?

"Aha again!, you say. "I can put a publisher policy into the GAC along with my assembly so that apps *are* updated automatically!" That's true, too, but now, like any of the machine-wide code replacement strategies of old, you're on the hook for an awesome responsibility: making sure that as close to 0% of apps, known to you or not, don't break. This is an awesome responsibility and one that takes MS hundreds of man-years at each new release of the .NET Framework. And even with those hundreds of man-years of testing, we still don't always get it right. If this is a testing responsibility that you're willing to live with, I admire you. Personally, I don't have the moral fortitude to shoulder this burden. For example, we do sign genghis.dll when we ship it so that folks can put it into the GAC if they want, but we make no promise of backwards compatibility between versions and therefore we do not ship publisher policy DLLs. Instead, we expect folks to use xcopy deployment and catch the problems at compile-time and test-time.

So, if the GAC represents such a massive burden, why do we even have it? It's for two things that I've been able to identify:

  1. Fixing critical bugs without touching the affected apps (and without breaking anything!)

  2. Sharing types at run-time between assemblies deployed separately

#1 is what you get when you install Windows hot fixes and service packs via Windows Update. A ton of design, implementation and testing time is spent to make sure that existing code won't break before shipping these fixes.

#2 is needed if you're going to be sharing types between assemblies that you can't deploy as a group but absolutely must keep to the same version of things. .NET Remoting peers are in this category, but only if they're deployed in separate directories so that they won't share the same set of types available via xcopy deployment. However, if .NET Remoting peers are deployed on difference machines, the GAC won't help you anyway as you'll be manually insuring the types are the same across machines. BTW, the responsibility of keeping multiple machines to the same set of types (and the same framework for hosting those types) spawned an entirely new way to talk between machines, i.e. web services, so .NET Remoting itself is something to avoid unless you can administer both ends of the pipe for simultaneous updates.

Another scenario that fits into #2 is the Primary Interop Assembly (PIA). A PIA is a COM interop assembly that's been pre-generated and dropped into the GAC so that everyone that adds a reference in VS.NET to mso.dll (the COM library for Office) gets office.dll instead (the .NET Office PIA). That way, everyone can talk to the same set of types instead of everyone getting their own incompatible interop types generated on the fly. However, PIAs are primarily a way to make sure that VS.NET has a central place to pick up the shared types without regenerating new incompatible types.

Notice that "Saving hard drive space" wasn't on my list of reasons to use the GAC. I just purchased a 60GB, 7200RPM hard drive for my laptop for a measly coupla hundred bucks. I don't want to hear about you jeopardizing the reliability of the applications on my machine to save a tiny percentage of that space. Hell, when I buy a HD, I give 50% of it over to apps anyway, so help yourself and keep my apps running by avoiding any machine-wide space for updating shared DLLs, including the GAC! Thanks.

Discuss (did I miss any reasons to use the GAC?)



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