Prevent Windows from using the PATH env variable when resolving dll dependencies?

Question

When loading a DLL (either dynamically with LoadLibrary/Ex or statically) is it somehow possible to prevent Windows from using the PATH environment variable to look up the DLL/other DLLs the DLL to be loaded depends upon?

The docs I could find for the Dynamic-Link Library Search Order seem to imply there is no way to prevent PATH from being searched, but maybe I'm missing something?

The reason I am asking this is that we would like to have a fail-fast scenario, when a (statically) linked DLL is missing from the application folder but is possibly present (different version) on the PATH.

Answer 1

For dynamically loaded dlls: the easiest way to control which one is loaded is to not invoke the search logic. The search logic is only invoked if a partial path is supplied - provide a fully qualified path to the dll when calling LoadLibrary and the call will fail if the Dll doesn't exist in that location.

For statically loaded dlls: Dlls that are part of an assembly are searched for ONLY in WinSxS and the application's folder. So, create a "dummy" assembly to hold the dll. Which is as simple as creating a .manifest file with contents like this:

<!-- dummyassembly.manifest -->

<assembly manifestVersion="1.0">
    <assemblyIdentity type="Win32" name="dummyassembly" version="1.0.0.0" processorArchitecture="x86"/>
    <file name="thedll.dll"/>
</assembly>

Add this code to any project that needs to use the exact dll only:

#pragma comment(linker, "/manifestdependency:\"dummyassembly'"\
                       " processorArchitecture='*' version='1.0.0.0' type='win32'\"")

and it will fail to load if the dll doesn't exist in the same folder.

Answer 2

You could change the PATH environment variable from code before loading your dlls. And then possibly restore it afterwords.