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.
The reason is that even though VBA strings are internally UTF-16, VB always converts them to ASCII before talking to the outside world (Declare
d 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.
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