Mixing 32-bit and 64-bit P/Invokes
32bit-64bit, c#, interop
Solution
The problem is not a .NET or P/Invoke. It is an OS issue. A 64-bit process can only load 64-bit DLLs. A 32-bit process can only load 32-bit DLLs. The magical Windows-on-Windows (or WoW) layer that lets 32-bit apps run on 64-bit Windows exists between the user-mode process (EXE and DLLs) and the kernel. There is no way to run a 32-bit DLL inside a 64-bit process. The WoW layer exists below that. (Basically WoW is a 32-bit wrapper around the 64-bit Win32 API, which marshals data and function calls between the 32-bit world of the process and the 64-bit world of the operating system.)
Your best/only option is to run your 32-bit and 64-bit components in separate processes and use some form of IPC to communicate. This has the added benefit of decoupling your core application from potentially unstable 3rd-party components. If a 3rd-party component crashes or misbehaves, it's simply a matter of re-starting the process containing that component.
Problem
I've run into a problem that I'm pretty sure I know the answer to, but I figured I'd at least ask and see if there was some "magic bullet" that might save me a huge headache. Here's the high-level view. I have a managed application. This application interfaces with hardware via third-party libraries from different vendors. I have full control over the consuming managed app and zero control over the hardware API libraries. Vendor A provides only a 32-bit native SDK. To allow us to use it on 64-bit systems, we marked the application to run in 32-bit mode. All was well. We are now integrating with Vendor B, which provides 64-bit-specific native API libraries on 64-bit machines. The 32-bit native DLL from Vendor B will not work on a 64-bit system (tried that). If I build a test harness running as 64-bit or AnyCPU, it works fine. If I mark it as 32-bit, it fails on the P/Invoke calls. It seems that Vendor A and Vendor B hardware are going to be mutually exclusive on 64-bit PCs, but I'm wondering if anyone has suggestions on how to possibly work around that.