64 to 32 bit Interop - how?

Question

I need to integrate some legacy 32-bit code - for which I don't have the source code, into a project in such a way that it can be called from a 64-bit .NET assembly. The original code is implemented as a 32-bit COM object in a DLL. Windows doesn't allow direct calls from 64 to 32-bit objects, so I'm looking for inspiration on how to deal with this situation.

How can a legacy 32-bit COM object be accessed from a 64-bit .NET assembly?

UPDATE: We discovered that the COM component was itself a wrapper around some ANSI C, which we founf the original source for. We were able to compile that in Visual Studio as a native 64-bit dll, and import that into .NET - sorry to move the goalposts!

Answer 1

The best approach is to make an out of process COM server that wraps your 32-bit DLL. You can then call this from 64bit code.

Here is an explanation of the basic concepts.

Answer 2

What you need to do is create two processes communicating with IPC. This way, one can be 32 bit, and one can be 64 bit. You need to create a 32 program which links with the COM object and exposes its API through some IPC mechanism such as a named pipe. This way your .NET program can access it from another process.

Answer 3

Check out this blog post. You can reference a 32 bit COM assembly from a 64 bit .NET application using a runtime callable wrapper. The short version is the following...

1. Use tlbimp.exe to create a 64 bit Runtime Callable Wrapper:

   tlbimp.exe foo.dll /machine:x64 /out:Interop.Foo.dll

2. Register the COM assembly (not the RCW) if you haven't already:

   regsvr32.exe foo.dll

3. Reference the RCW (eg. Interop.Foo.dll) from your application. Change your Build Configuration to x64 and let 'er rock.