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PROCESSOR_ARCHITECTURE and Visual Studio Debugging

For years I’ve labored under the impression that the PROCESSOR_ARCHITECTURE environment variable was an indication of the bitness of the current process. Even after comprehending that in a 64-bit process this variable yields AMD64, even when the machine is fitted with an Intel processor (because AMD invented the 64-bit extensions), I’ve written several components in the past that rely on PROCESSOR_ARCHITECTURE, and never expended too much thought about it. I mean, this has to be the simplest and most reliable way to do it, right?

However, a colleague recently asked me if I had any idea why he couldn’t debug an assembly in Visual Studio. He was getting a BadImageFormatException, a surefire indicator that something was trying to load a 64-bit binary into a 32-bit process (or vice-versa). The code he was trying to debug in this case was an add-in to Excel, so he’d configured the project in Visual Studio to “Start external program”, and pointed to Excel, but Excel wouldn’t start when hitting F5, and the logs revealed the exception. Starting Excel directly with the add-in configured worked just fine.

As it turned out, we were using PROCESSOR_ARCHITECTURE in this assembly to determine which version to load (x86 or x64) of a lower-level native dependency. In production, this would work just fine, but when debugging from Visual Studio, the x64 version of the native component was being loaded, which was not making the x86 Excel process very happy at all.

To simplify the diagnosis, I created a new console application in Visual Studio, set the project to debug into C:\Windows\SYSWOW64\cmd.exe (the 32-bit command prompt), and hit F5. Sure enough, from the resulting shell, echo %PROCESSOR_ARCHITECTURE% was yielding AMD64, in a 32-bit process! Something was clearly not quite right with this picture.

As it transpires, when debugging this way in Visual Studio, the external program starts in an environment that’s configured for the wider of the bitnesses of the startup project’s current build platform and the started executable. In my simple console app, if I set the platform to x86 instead of the default Any CPU, the resulting command prompt now yielded x86 instead of AMD64 for PROCESSOR_ARCHITECTURE. But if I instead debugged into C:\Windows\System32\cmd.exe, PROCESSOR_ARCHITECTURE yielded AMD64 regardless of the project’s current build platform.

Since the code in question here was in a managed assembly, I switched the PROCESSOR_ARCHITECTURE check to instead consider the value of IntPtr.Size (4 indicates x86, 8 indicates x64). Hopefully when we go to .NET 4.0 we’ll be able to take advantage of Environment.Is64BitProcess and Environment.Is64BitOperatingSystem for this kind of thing, at least from managed code.

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