VBA to C++ DLL String Passing Issue on Windows 11: Receiving Empty String

I expected to be be able to pass a VBA string to a C++ DLL function.

Here is the C++ code (compiled in Visual Studio Community 2022 as a x64 Debug):

// dllmain.cpp : Defines the entry point for the DLL application.
#include "pch.h"
#include <Windows.h>

extern "C" {

    __declspec(dllexport) void ShowMessage(LPSTR message) {

        MessageBoxA(NULL, message, "DLL Message", MB_OK);

    }

}

And the Excel VBA code (Excel Version 2307 Build 16.0.16626.20170, 64-bit):

'Test function
Declare PtrSafe Function ShowMessage _
    Lib "MyFunTest.dll" (ByVal message As String)


Function VBShowName()

    Dim str_message As String
    str_message = "Hello World"
    ShowMessage (str_message)
     
    VBShowName = str_message
    
End Function

When I compile the DLL and run in debug, Visual Studio shows that message in C++ has a value of "" (as if I had not initialized the str_message variable in VBA) as soon as I enter the function. I see the VBA function returns the value of Hello World so I think I am storing the string correctly in VBA.

I need help understanding how to pass strings from Excel VBA to a C++ DLL (I'm upgrading old C++ code from 2010 that used to work with 32-bit Excel).

I've read through a few articles (Passing Strings to a DLL Procedure,Access DLLs in Excel, Pass String from VBA to C DLL to Modify as a Parameter and Return, Passing strings from VBA to C++ DLL) to troubleshoot the problem, but have not been able to pass the string.

VBA passes "" to the C++ DLL regardless of what I've tried.

I also tried using the BSTR type for the input to the DLL function, but got the the same "" values.

// dllmain.cpp : Defines the entry point for the DLL application.
#include "pch.h"
#include <Windows.h>
#include <oleauto.h>


extern "C" {

    __declspec(dllexport) void ShowMessage(BSTR message) {

        MessageBoxW(NULL, message, L"DLL Message", MB_OK);

    }

}

I tried LCSTR as well, but didn't get the string in C++. Instead in showed '\f'. Here's the C++ code:

    extern "C"  __declspec(dllexport) void __stdcall passByNarrowVal(LPCSTR s)
    {
        USES_CONVERSION;
        MessageBox(NULL, A2W(s), L"Pass by Narrow Val", MB_OK | MB_ICONINFORMATION);
    }

Here's the VBA Code:

Declare PtrSafe Function passByNarrowVal _
    Lib "MyFunTest.dll" (ByVal s As String)

Dim tline As String
tline = "Hello World"
passByNarrowVal (tline)