How do I install both 32-bit and 64-bit versions of a COM DLL and "auto-select"?

Question

We've got a DLL (a COM server) that will compile fine in 32-bit and 64-bit, but the DLL uses the same CLSID and AppID for the 32-bit version and the 64-bit version. Is this OK or does this have to change?

I'm asking this because apparently on a 64-bit machine, we can't register the 32-bit version and the 64-bit version together. It would be nice if 32-bit client applications could automatically use the 32-bit DLL, and 64-bit client applications could automatically use the 64-bit DLL.

On a related note, we have the source code and Visual Studio 2005 project file for a client application ... how do we compile a 32-bit and 64-bit version of the same application? It's a C# application, and it includes a reference to our COM server DLL like this:

<ItemGroup> <COMReference Include="ComServer">

<Guid>{C1FADEA6-68FD-4F43-9FC2-0BC451FA5D53}</Guid>

<VersionMajor>830</VersionMajor> <VersionMinor>0</VersionMinor>

<Lcid>0</Lcid> <WrapperTool>tlbimp</WrapperTool> <Isolated>False</Isolated>

</COMReference> </ItemGroup>

If it turns out that we need a separate CLSID for 64-bit, how do we make this reference "only for the 32-bit configuration" in Visual Studio? Or do we have to have separate projects with the same source code: one that refers to 32-bit DLL, and the other that refers to 64-bit DLL?

Answer 1

Both versions can (and indeed should) use the same GUIDs for everything.

On a 32-bit machine you can't register or use the 64-bit DLL, so there's no problem there. The 64-bit DLL simply doesn't enter the picture.

On a 64-bit machine the 64-bit DLL gets registered in HKLM/Software/Classes/CLSID (etc.) and the 32-bit DLL gets registered in HKLM/Software/Wow6432Node/Classes/CLSID. (I wonder where you got the advice that you can't register the 32-bit DLL on a 64-bit machine...) A 32-bit client running on the 64-bit machine will look in the normal place in the registry, but the OS will silently redirect it to the Wow6432Node key instead.

Answer 2

This is dealt with inside Windows with a feature called 'Registry redirection' On the 64-bit version of Windows, a 32-bit program gets a different view of the registry. Any access to the HKCR alias or the HKLM\Software root is redirected for the kind of keys that a COM server uses. A 32-bit program actually sees the key values stored in HKLM\Software\Wow6432Node. You can see it with Regedit.exe

This is normally taken care of by an installer, a VS Setup project has the TargetPlatform property for example. If you want to make the COM server available in both 32-bit and 64-bit mode then you should use two installers. Or a 64-bit installer that writes both sets of keys. Having a COM server that can handle both is very unusual in the olden days. But not unheard of when you implemented in .NET for example.