Can an x64 application use x86 assemblies - and vice versa?

Question

My application is built as a x64 application. After moving to VS2010 I got some problems which seems to be related to some x64/x86 mismatch in referenced dlls. Now I'm moving to target .NET4, and I get even more similar problems.

My question is: What precautions do I need to take regarding mixing x64 and x86. Can it be done at all? I thought x64 applications should be able to use x86 dlls without problems. No? What about the other way? Can a x86 application reference an x64 dll - as long as it is being run on an x64 platform? What are the pitfalls I need to be aware of?

Solution 1

No, a 64-bit process can only load 64-bit DLLs and a 32-bit process can only load 32-bit DLLs. What you're probably thinking of is that a 64-bit operating system can run 32-bit processes.

The main issue with .NET is that - prior to VS2010 - executable projects defaulted to "AnyCPU" which means it would load in the "native" format of the OS it's running on (so 32-bit for 32-bit versions of Windows and 64-bit for 64-bit versions of Windows). The problem with that is that if you tested your application on 32-bit Windows (say) then it could break if you load 32-bit DLLs and tried to run on 64-bit Windows.

In VS2010, they defaulted all executable projects to be "x86" (that is, 32-bit) by default which (for the most part) mitigates the problem.

Solution 2

You can run x86 apps on a 64 bit OS using WOW32 emulation. Some pitfalls that I have encountered - you can't mix and match 32/64 in the same process. So if you intend to run IIS as 64 all the assemblies need to be 64 otherwise you'll have to run In 32 bit mode. 64 bit helps some apps more than others. Running SQL sever's 64 bit version provides several advantages over the 32 bit version, biggest advantage being that yo can install more than 4 GB of memory on the target server and that SQL will be able to use more than 4 GB of memory. It does not benefit IIS as much because IIS typically can't use more than 3 gb of memory. My advice would be to make sure your SQL server/os/version are 64 if possible. It's not going to make a. Huge dfference if the other servers are 64 but typically it's easier to work with and find 32 bit versions.