Dll dependency version conflict

Question

I am using C++ with Visual Studio 2008 Express.

We are supplying a binary-only Windows library to a client, which itself uses a number of other libraries. Therefore we shipped both our dll file, along with the dll files that we use. Now the issue is that our client uses some of the libraries that we also use, but in another version. Therefore he can not use our library, since the libraries we both depend on are incompatible.

Technically I think it should be possible that both dependency versions are loaded into the process space. However, I am unsure how to do this, as both their application, as well as our dll look for the same dependency dll file name. Can anyone tell me what the best/cleanest way to deal with this problem is?

Thanks!

Answer 1

You could solve this kind of problem using a (new) additional DLL you would deliver and that would take care of handling the versions conflict (at runtime) - being a kind of proxy between your app and its dependencies.

An alternative would be to use the Windows Forwarded Libraries mechanism .

Forwarders are a handy way to accommodate functionality moving from one DLL to another

You can use several ways to declare forwarders, such as a module definition ( .def ) file and a #pragma :

#pragma comment(linker, "/export:function=otherdll.function")

Answer 2

Generally speaking, it won't work. This is due to the fact that the third party DLL versions might interfere with each other when loaded into memory. One example could be if there is an exclusive resource like eg a file in a specific directory. Or a specific device. The problem is, nobody knows probably not even the manufacturer of the 3rd party DLLs - so extensive testing is necessary.

But maybe you're lucky and it works anyway. My recipe:

1. Put your DLL "DTAG.DLL" and all needed DLLs in a subdirectory of the applications directory with a fixed name eg "DTAG_LIB".
2. Write a import library by hand (there are other possibilities using DELAYLOAD). In that library load your DLL with LoadLibraryEx. Provide an absolute path ending with "DTAG_LIB\\DTAG.DLL" and the flag LOAD_WITH_ALTERED_SEARCH_PATH. Windows will then load your DTAG.DLL from this directory and all needed DLLs from that directory also. Don't set the PATH to "DTAG_LIB"!
3. Your customer has to link against your manual import lib.