C# compiling for 32/64 bit, or for any cpu puzzler

Question

This question relates to these previous question on SO

Any CPU question 1 and Any CPU Question 2

I have a application which was originally built on Win XP using Visual Studio 2005 (don't laugh!). This app calls into our win32 C++ dll. The C# components which call the C++ dlls were built with the "Any CPU" configuration and have been happily working on Win XP without any issue.

We are now moving to Win 7 and the release version of our app (which was built on Win XP with VC 2005) works fine. However with the roll out of win 7 to our users we have now taken the opputunity to move to VS 2010 and I have built the C# components on win 7 with VC 2010 but now when running this version I get lots "unable to load abc.dll" where abc.dll is our win32 c++ components.

I understand that recompiling the C# assemblies with x86 config will solve the problem but what I don't understand is how the release version c# assemblies built with Win-XP/Visual studio 2005 (Any CPU config) are able to run on Win 7 without any issues? Surely these C# assemblies built with "Any CPU" should JIT to 64 bit code when loaded in Win 7 and cause BadImageFormatException or other errors because they call Win32 C++ dlls.

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UPDATE : I have Some more information that have been requested in the comments below.

1. On my Windows 7 box I right click on my computer and look at the properties. The System information says "System Type : 64 bit operating system" confirming this is a Win64 OS.

2. Opening up the solution in VC2005 on Windows XP When viewing the configuration manager for the solution I can confirm ALL the C# projects are platform type "Any CPU".

3. When running the release build (which was done on VC2005/win xp) on 64bit Win 7 machine, I task manager shows the image name as "Test.exe *32", this confirms it is jit'd and loaded into 32bit process.

Answer 1

Under Win7, the process is 64-bit. 64-bit processes cannot have 32-bit DLLs in them, that's a pretty fundamental design limitation of Windows.

And the fact that this managed code EXE calls into a 32-bit DLL is not obvious from the get-go - P/Invoke, as well as COM interop, works via late binding. So the EXE is loaded, the loader does not check the dependencies - for one thing, the dependencies might be conditional - then the DLL loading time comes, wackiness ensues.

So yeah, if you have managed code with known 32-bit dependencies, you better specify a 32-bit CPU at compile time. Or recompile the C++ parts to 64 bits, that's also an option.

Answer 2

I can confirm that you should get BadImageFormatException. Is it possible that the compilation on XP was monkeyed with badly enough that it's not actually building AnyCPU but rather builds x86 labelled as AnyCPU. I can also confirm it's possible to monkey with a project file badly enough that it would do that and the project upgrade component chokes on that.