从Excel VBA调用的C ++ DLL仅在Visual Studio中的调试实例中有效

Question

I have created a 64 bit dll in C++. There is an Excel macro that can call it by listing the full path to the dll. However, it only works if I open the containing Excel file by right clicking on the C++ project and selecting Debug->Start New Instance. If I open the Excel file directly from the file explorer and try to run the dll, it gives me a "File not found" message box that it can't find the dll, with the path listed. It gives the same error in Release mode. However, when I check the path, the dll is present and seems to compile fine to the specified location.

This hasn't happened with other dlls I have made. A similar dll can be called whether I open it from the file explorer or a debug instance. There isn't any apparent reason for the difference. They both call other dlls, have namespaces, and have classes, but the problem seems to be occurring before it even reaches the code.

C++ (test function in the DLL):

int test(int &x)
{
    return x+8;
}

VBA code:

Private Declare PtrSafe Function test Lib _
"[path to dll]" (ByRef x As Long) As Long

'  Use function on worksheet
 Function testThis(x As Long) As Long
    On Error GoTo Catch
    testThis = test(x)
Catch:
    If Err <> 0 Then
        MsgBox (Err.Description)
    End If
    End Function

If I enter "2", for instance, it should return "10" in all cases. This happens from the debug Excel instance started through Visual Studio only, but gives me an error message that it can't find the DLL when the file is opened directly. I have no idea why.

Edit: The path is correct in VBA. Do you honestly think I haven't checked that? It's private info and if it were wrong it wouldn't work in the debug Excel instance.

Dependency Walker gives the following errors: Error: At least one required implicit or forwarded dependency was not found. Error: At least one module has an unresolved import due to a missing export function in an implicitly dependent module. Error: Modules with different CPU types were found. Error: A circular dependency was detected. Warning: At least one delay-load dependency module was not found. Warning: At least one module has an unresolved import due to a missing export function in a delay-load dependent module.

I'm looking into what these could mean, but haven't used this before and am a C++ and VBA novice.

Answer 1

I couldn't identify an issue with dependencies, but Excel does need some help finding them in general. In order to get any dependent DLLs to work at all when called from Excel, including this case, here is one way to do so. Let's say the dependent DLL I want to call directly is "A" and an installed one it depends on is DLL "B". In order to call any function from the dependent DLL A, I first have to call any function from the independent DLL B. It doesn't matter which function or even if the function I'm calling from Excel directly depends on the DLL B. Using this hack, I can get all other functions in A to run, provided there are no unrelated issues. If I open the Excel file directly from the File Explorer and try to run the DLL without this hack, it gives me assorted errors like VALUE or "cannot find the dll".

I also cleaned up the dependencies in the projects Properties->Input->Additional dependencies, to something like B.lib;%(AdditionalDependencies), but this didn't have an effect. It just looks better.

I haven't been able to get LoadLibrary to work yet, but that was suggested in the comments.