Argument not taken into account when using C++ dll in excel - c++

Question

Hello I am trying to create a simple C++ dll to be used in Excel.
Here is what I did:
.cpp file:
double _stdcall Test(double z)
{
return z+2.0;
}
.def file:
LIBRARY
EXPORTS
Test
In visual studio:
Project Properties > Configuration properties > Command > "Path/EXCEL.EXE"
Configuration manager > Platform > x64
In VBA:
Declare PtrSafe Function Test Lib _
"Path\MyDLL" (ByVal z As Double) As Double
But when I call Test(2) in Excel, it returns 2, and not 4. It seems like argument are seen as 0 alway (actually if I output the value of z in a file while calling the function, it is 2.122e-314).
Any inputs would be greatly appreciated.
Thanks
Edit 1:
If I change the argument and return value to int and in VBA to Long, Test(2) returns 3.
Anyone has an idea why the argument is 1 for int?

Answer 1

Apologies: Would have posted this as a comment but can't yet:
First: Your function may be exported as a C++ mangled name.
Use extern "C" to prevent name mangling. There are other ways to do this, but this is most direct. However if the Test function is successfully being called I'm not sure this is the issue.
Using extern "C" your C function would look like this.
extern "C"
double _stdcall Test(double z)
{
return z+2.0;
}
Second: Are you absolutely certain you are picking up the right DLL, not an old one which is missing your fixes? Try changing the function name to something more specific in your source and in the Excel VBA wrapper.
Third: If you are not running on a 64bit Excel system remove the PtrSafe declaration from your function specification in VB and recompile as 32-bit.
Fourth: Do not try to call your DLL directly from an Excel Cell. Wrap the function in VBA (example below) or use the CALL function.
The code below is an example how to wrap write a VBA cell function.
Public Function CellFunction_Test(x As Double) As Double
'' Tells excel to call this function only if the inputs change
'' and not on every recalculation
Application.Volatile False
'' Set an error handler, good practise for more complicated functions you might write later
On Error GoTo CellFunction_Test_EH:
'' Call into the DLL
CellFunction_Test = Test(x)
Exit Function
'' Error Handler
CellFunction_Test_EH:
CellFunction_Test = "Error - " & Err.Description
End Function