How to use `/DELAYLOAD` on a DLL from another DLL - c++

Question

I have a solution with two DLLs. The first one is the "main" DLL. It happens to be an ODBC driver, but I think that is not important for this question.
The second DLL contains all the UI logic for the first one. As the UI is not always needed, I want to use the /DELAYLOAD feature which explicitly says:
The delayed loading of a DLL can be specified during the build of
either a .EXE or .DLL project.
The main DLL's project correctly references the UI ones. If I don't use /DELAYLOAD, everythin works just fine. The two DLLs will be installed into the same directory, so I thought loading one DLL from within the other should be easy. But apparently, it's not.
As soon as the first function from the UI DLL is called, the application (any ODBC client in my case) crashes.
GetLastError() yields 126 which apparently means that the target DLL could not be found in any of the search paths.
And indeed, according to this answer LoadLibrary() does have a look into the directory of the calling executable, but not into the one of the currently executed DLL. I'm assuming /DELAYLOAD is also just using LoadLibrary() under the hood, is that correct?
If I copy the executable into the installation directory of my driver, it works just fine, which proves my assumption that it just doesn't look in the current DLL's directory.
Appart from that, I was also able to make it run by calling
LoadLibrary(L"C:\\absolute\\path\\to\\UI.dll");
just before the first function of the UI DLL is loaded.
I was also able to determine this path programmatically using
wchar_t buffer[512];
GetModuleFileName(hThisDLL, buffer, sizeof(buffer));
But then I would have to cover every single UI call with this logic. So I wouldn't see much advantage anymore that /DELAYLOAD has over the "old-school" way of using LoadLibrary() and GetProcAddress().
Question
Is there a simple way to make /DELAYLOAD find the target DLL from another DLL in the same directory?

Answer 1

There is. My suggestion would be to create a delay-load-failure hook function.
https://learn.microsoft.com/en-us/cpp/build/reference/failure-hooks?view=vs-2019
Basically, you write a function inside your main DLL that gets notified in the event of a delay load failure. In that function, when the given code indicates failure, you try manually calling LoadLibrary with of a path consisting of the folder in which your main DLL resides plus the name of the DLL that failed to load
How you get the your main DLL from within your main DLL is up to you. There are many ways.
Something like this:
FARPROC WINAPI delayHook(unsigned dliNotify, PDelayLoadInfo pdli)
{
FARPROC fpRet = NULL;
switch (dliNotify)
{
case dliStartProcessing:
break;
case dliNotePreLoadLibrary:
break;
case dliNotePreGetProcAddress:
break;
case dliFailLoadLib:
{
std::string newPath = GetMyModulePath();
newPath += "\\";
newPath += pdli->szDll;
fpRet = reinterpret_cast<FARPROC>(::LoadLibrary(csDir));
}
break;
case dliFailGetProc:
break;
case dliNoteEndProcessing:
break;
default:
break;
}
return fpRet;
}
//
// Set access to our delay load hook.
//
PfnDliHook __pfnDliFailureHook2 = delayHook;