64-bit Apps / 32-bit Assemblies

Our team was working late the other night and got tripped up on how "Any CPU" works with 64-bit operating systems. I had developed a simple command-line utility that used some third-party assemblies that were installed in the GAC. Everything worked great on my local machine until I deployed it into the build environment, where the app simply blew up and reported an awkward "FileNotFoundException".

At a quick glance, it seemed obvious: the 3rd party assemblies weren't in the GAC. My head snapped back a bit when C:\Windows\Assembly showed that the exact versions I needed were in fact installed. I double checked my project references and painfully grudged through security policies, FusionLog files and web.config settings and with each attempt I became increasingly aggravated and annoyed. When I copied the 3rd party assemblies out of the GAC and into my utility's folder and got a BadFormatException, I realized that the only difference between my machine and the build environment was the 64bit operating system.

On a hunch, I recompiled my utility to target "x86" and suddenly everything worked. The team was baffled -- why didn't "Any CPU" work?

What does “Any CPU” mean?

The .NET Framework inspects the meta data of your dll or exe to determine how to load it. By default, "Any CPU" creates a native process to that OS. In other words, 32 bit process for 32bit OS and 64 bit process for 64bit OS. When a 32bit application (x86) runs on a 64bit OS, it runs as a WoW64 emulated 32bit process (Windows on Windows).

The following table outlines how the Visual Studio Build Configuration is interpreted between 32bit and 64bit OS.

	Any CPU	x86	x64
32 bit OS	Native (32 bit)	32 bit process	N/A
64 bit OS	Native (64 bit)	WoW64 process (emulated 32 bit)	64 bit process

The real gotcha is that your referenced assemblies must match the process space of the application they're being loaded into. Any assembly that is compiled as "Any CPU" is considered "neutral" and can be loaded into either a 32bit or 64bit process.

How do I determine what platform my assembly requires?

There are a few ways to determine if your assembly is Any CPU, 32bit or 64bit:

Global Assembly Cache

If your assembly is in the GAC, you can find this information in the Processor Architecture column.

Corflags

The corflags.exe tool that ships with the .NET SDK is a very powerful and dangerous tool. Just supplying the name of your exe or assembly to corflags shows some vital PE meta information. More great information can be found here: http://blogs.msdn.com/joshwil/archive/2005/05/06/415191.aspx

Incidentally, instead of recompiling the application with the “x86” target, I could simply have hacked the header using corflags:

corflags myapp.exe /32bit+

However, hacking the header of the assembly using corflags will invalidate the digital signature. Although you can always re-sign the assembly using the strong-name (sn) utility, it’s easier to just recompile and deploy.

Cheers.