Force x86 CLR on an 'Any CPU' .NET assembly

Question

In .NET, the 'Platform Target: Any CPU' compiler option allows a .NET assembly to run as 64 bit on a x64 machine, and 32 bit on an x86 machine. It is also possible to force an assembly to run as x86 on an x64 machine using the 'Platform Target: x86' compiler option.

Is it possible to run an assembly with the 'Any CPU' flag, but determine whether it should be run in the x86 or x64 CLR? Normally this decision is made by the CLR/OS Loader (as is my understanding) based on the bitness of the underlying system.

I am trying to write a C# .NET application that can interact with (read: inject code into) other running processes. x64 processes can only inject into other x64 processes, and the same with x86. Ideally, I would like to take advantage of JIT compilation and the Any CPU option to allow a single application to be used to inject into either x64 or x86 processes (on an x64 machine).

The idea is that the application would be compiled as Any CPU. On an x64 machine, it would run as x64. If the target process is x86, it should relaunch itself, forcing the CLR to run it as x86. Is this possible?

Answer 1

You can find out how an application will run and change it statically using the CorFlags application. To find out how the application will run, use:

corflags <PathToExe>

To change how the application will run, use:

corflags /32bit+  <PathToExe>

This will make the EXE file run as a 32-bit process. The information about how the assembly should run is stored in the PE header. See Stack Overflow question How to find if a native DLL file is compiled as x64 or x86?.

If you want to inject code at run time, you have to write a .NET profiler in C++/COM. See .NET Internals: The Profiling API and Profiling (Unmanaged API Reference) for more details.

You'll need to implement the JitCompilationStarted callback and do your work there. If you go in this direction, you'll have to build the injecting DLL file both as x86 and x64. The native DLL files will be loaded by the CLR once the following environment variables will be set:

Cor_Enable_Profiling=0x1
COR_PROFILER={CLSID-of-your-native-DLL-file}

If you have it set correctly then the 64-bit version will 'see' the 64 bit processes and the 32-bit version will 'see' the 32-bit processes.

Answer 2

It has been a while since I tried this, but I believe that the bitness of the process that calls the assembly determines whether it will be JITed as x86 or x64.

So if you write a small console application and build it as x86, and another as x64, running one or the other will cause other assemblies loaded into the process to run as 32 or 64 bit. This, of course, assumes you are running on a 64 bit machine.