Calling C++ function from Excel and VBA using DLL created in C++

Question

I created a DLL containing a function named "koduj". Calling this function by using it inside an Excel worksheet cell returns the desired result. Calling "koduj" from VBA returns wrong answer.

koduj needs two arguments: string nr_id and integer x1. It calculates sum of nr_id's letters in ASCII representation and adds x1. Calculated sum is than returned.

I was following instructions found here.

Here's my .cpp sourcefile:

#include<Windows.h>
#include<string>
using namespace std;


//Convert BSTR to wstring for convenience
wstring BSTR_to_wstring (BSTR text){
    return wstring(text, SysStringLen(text));
}

//Calculate sum of letters in ASCII representation
int ASCII_sum (wstring ws){
    int sum = 0;
    for (unsigned int i = 0; i < ws.length(); i++)
        sum += ws[i];
    return sum;
}

//"koduj" function
int _stdcall koduj (BSTR nr_id, int & x1){
    wstring ws_nr_id = BSTR_to_wstring(nr_id);
    return ASCII_sum(ws_nr_id) + x1;
}

Here's my VBA function declaration:

Declare Function koduj _
Lib "<dll_directory_and_full_name>" (ByVal x As String, ByRef y As Integer) As Integer

By writing:

=koduj("aaa";1)

Inside a worksheet cell I get desired result (292)

Debugging this VBA code:

Sub test()

Dim a As Integer
a = koduj("aaa", 1)

End Sub

reveals wrong result (a = 24930)

I believe my C++ code is fine, as it works properly when called from Excel's worksheet.

Solution

The reason is that even though VBA strings are internally UTF-16, VB always converts them to ASCII before talking to the outside world (Declared functions, file input/output). So when you Declare a parameter As String, VBA automatically converts the string and passes it out as ASCII. The matching parameter type on the C++ side should be LPSTR or LPCSTR.

If you want to use BSTR on the C++ side, you need to also create an IDL file for that function, compile it into a TLB and reference the TLB from VBA, only then VBA will respect and use BSTR.

Another problem is that C++'s int translates to VBA's Long.

The reason why it works when called from Excel sheet is that apparently Excel ignores the VBA rules for string conversion. I believe this to be a bug.