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)
