Loading a 32-bit or 64-bit side-by-side COM DLL depending on the bitness with which the application runs - com-interop

Question

I have a .NET application that uses a COM DLL, of which there is both a 32bit and a 64bit version. I have written two application manifests that make side-by-side COM interop work on either 32 Bit or 64 Bit. Here the 32-bit version:
<?xml version="1.0" encoding="utf-8"?>
<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
<assemblyIdentity name="MyApp" version="1.0.0.0" type="win32" />
<dependency>
<dependentAssembly>
<assemblyIdentity
type="win32"
name="MyCOMDll_32.dll"
version="1.2.3.4"
processorArchitecture="x86"
publicKeyToken="0000000000000000"
language="*" />
</dependentAssembly>
</dependency>
</assembly>
However, maintaining two manifests leads to the loss of portability: you need to decide which version to use when you install the application. And the 64-bit application can no longer be run in 32-bit mode.
Is there a possibility to get the .NET application to load the correct 32-bit or 64-bit DLL depending on the bitness under which it runs?
I have tried using two dependency elements, one with <assemblyIdentity processorArchitecture="x86" .../> and one with <assemblyIdentity processorArchitecture="amd64" .../>, but that results in an application configuration error.
I'd be very grateful for answers.
Regards,
Moritz

Answer 1

I have not found a way to do this with an application manifest.Therefore, I dropped the application manifest in favor of a programmatic solution using the Activation context API.
This solution has been adapted from http://support.microsoft.com/kb/830033/en-us (where the field cookie must be an IntPtr not a uint).
I have replaced the inner part of the EnsureActivationContextCreated() method by
if (!contextCreationSucceeded)
{
string manifestLoc = Environment.Is64BitProcess
? "MyCOMDll_64.dll.manifest"
: "MyCOMDll_32.dll.manifest";
myComActivationContext = new NativeMethods.ACTCTX();
myComActivationContext.cbSize = Marshal.SizeOf(typeof(NativeMethods.ACTCTX));
myComActivationContext.lpSource = manifestLoc;
// Note this will fail gracefully if file specified
// by manifestLoc doesn't exist.
hActCtx = NativeMethods.CreateActCtx(ref myComActivationContext);
contextCreationSucceeded = hActCtx != Failed;
}