Passing arguments from VBA to DLL

[mkluwe]

Hi!

I just took the first few steps in writing a little DLL that should
be called from Excel/VBA, and I stumbled upon some wierd behaviour.

The function in my DLL is declared as

int __stdcall foo( const char *t );

and is used in VBA via

Declare Function foo Lib "C:\foo\foo.dll" (ByVal t As String) As Long

I noticed that foo didn't work internally as I expected and added
writing *t to a file on each call of foo as a debugging measure.
According to this output, only the first character of String t seems
to be passed to foo when called directly from Excel (putting
=foo("xyz") in a cell).

However, if I wrap foo with

Function call_foo(t As String) As Long
call_foo = foo(t)
End Function

and use the formula =call_foo("xyz"), everything works ok.

Any ideas?

Regards,
Matthias

PS. Any other hints to "VBA calls DLL" documentation are appreciated

 

[sjoo]

hi,

you can't directly call the dll function on the worksheet.
if you like to do, you have to make XLL library that is a kind of dll.
XLL is a special programming model for EXCEL.

please refer this
http://support.microsoft.com/defaul...port/kb/articles/q178/4/74.asp&NoWebContent=1

 

[mkluwe]

Hi!

I just took the first few steps in writing a little DLL that should
be called from Excel/VBA, and I stumbled upon some wierd behaviour.

[...]

I noticed that foo didn't work internally as I expected and added
writing *t to a file on each call of foo as a debugging measure.
According to this output, only the first character of String t seems
to be passed to foo when called directly from Excel (putting
=foo("xyz") in a cell).

you can't directly call the dll function on the worksheet.

In fact, I can, but lacking the correct result. Any guesses for the
technical reason?

if you like to do, you have to make XLL library that is a kind of dll.
XLL is a special programming model for EXCEL.

please refer this
http://support.microsoft.com/defaul...port/kb/articles/q178/4/74.asp&NoWebContent=1

Sounds interesting, but I don't have the mentioned "Microsoft Excel 97
Developer's Kit" available. Plus, Excel 97 is pretty old. What do I
have to do when developing for more recent versions?

Regards,
Matthias

 

[NickHK]

does this help ?
http://support.microsoft.com/kb/106553/EN-US/

NickHK

Hi!

I just took the first few steps in writing a little DLL that should
be called from Excel/VBA, and I stumbled upon some wierd behaviour.

[...]

I noticed that foo didn't work internally as I expected and added
writing *t to a file on each call of foo as a debugging measure.
According to this output, only the first character of String t seems
to be passed to foo when called directly from Excel (putting
=foo("xyz") in a cell).

you can't directly call the dll function on the worksheet.

In fact, I can, but lacking the correct result. Any guesses for the
technical reason?

if you like to do, you have to make XLL library that is a kind of dll.
XLL is a special programming model for EXCEL.

please refer this

http://support.microsoft.com/defaul...port/kb/articles/q178/4/74.asp&NoWebContent=1

Sounds interesting, but I don't have the mentioned "Microsoft Excel 97
Developer's Kit" available. Plus, Excel 97 is pretty old. What do I
have to do when developing for more recent versions?

Regards,
Matthias

 

[mkluwe]

Hi!

:
[...]

I noticed that foo didn't work internally as I expected and added
writing *t to a file on each call of foo as a debugging measure.
According to this output, only the first character of String t seems
to be passed to foo when called directly from Excel (putting
=foo("xyz") in a cell).

you can't directly call the dll function on the worksheet.

In fact, I can, but lacking the correct result. Any guesses for the
technical reason?

if you like to do, you have to make XLL library that is a kind of dll.
XLL is a special programming model for EXCEL.

please refer this

http://support.microsoft.com/defaul...port/kb/articles/q178/4/74.asp&NoWebContent=1

Sounds interesting, but I don't have the mentioned "Microsoft Excel 97
Developer's Kit" available. Plus, Excel 97 is pretty old. What do I
have to do when developing for more recent versions?

does this help ?
http://support.microsoft.com/kb/106553/EN-US/

No, sorry. The example DLL and VB code looks like mine, and I have no
problems when calling the DLL from VBA functions or macros. The problem
arises when calling it from wihtin Excel.

Regards,
Matthias

 

[NickHK]

I'm no C expert at all, but..
I would assume it has to do with how VBA sends the string to your DLL. In
VBA, internally, it is held as Unicode in a BSTR, which works fine when
passed ByVal to your function. IIRC, you are actually passing a pointer to a
pointer of the string.
However, going directly from the worksheet, your are not actually passing
the same thing (possibly not even in a BSTR) and hence only the first
character is resolved instaed of the whole string.

As a trivial example, using this Windows calls
Public Declare Function lstrlenA Lib "kernel32" (ByVal lpString As String)
As Long
in a VBA function gives lstrlenA("qwerty") equal to 6. Correct.

From the worksheet, lstrlenA("qwerty")=1. Wrong

So I would guess you have to give VBA chance to create a variable so it can
pass the string correctly. i.e. create a dummy VBA function to call each of
your DLL functions that invlove strings.

NickHK

Hi!

sjoo wrote:
:
[...]

I noticed that foo didn't work internally as I expected and added
writing *t to a file on each call of foo as a debugging measure.
According to this output, only the first character of String t
seems
to be passed to foo when called directly from Excel (putting
=foo("xyz") in a cell).

you can't directly call the dll function on the worksheet.

In fact, I can, but lacking the correct result. Any guesses for the
technical reason?

if you like to do, you have to make XLL library that is a kind of
dll.
XLL is a special programming model for EXCEL.

please refer this
http://support.microsoft.com/defaul...port/kb/articles/q178/4/74.asp&NoWebContent=1

Sounds interesting, but I don't have the mentioned "Microsoft Excel 97
Developer's Kit" available. Plus, Excel 97 is pretty old. What do I
have to do when developing for more recent versions?

does this help ?
http://support.microsoft.com/kb/106553/EN-US/

No, sorry. The example DLL and VB code looks like mine, and I have no
problems when calling the DLL from VBA functions or macros. The problem
arises when calling it from wihtin Excel.

Regards,
Matthias

 

[mkluwe]

Hi!

sjoo wrote:
:
[...]

I noticed that foo didn't work internally as I expected and added
writing *t to a file on each call of foo as a debugging measure.
According to this output, only the first character of String t
seems
to be passed to foo when called directly from Excel (putting
=foo("xyz") in a cell).

you can't directly call the dll function on the worksheet.

In fact, I can, but lacking the correct result. Any guesses for the
technical reason?

I'm no C expert at all, but..
I would assume it has to do with how VBA sends the string to your DLL. In
VBA, internally, it is held as Unicode in a BSTR, which works fine when
passed ByVal to your function. IIRC, you are actually passing a pointer to a
pointer of the string.

The C function accepts a pointer to type char; you can interpret this
as a pointer to the string in memory, if you like to.

[...]

As a trivial example, using this Windows calls
Public Declare Function lstrlenA Lib "kernel32" (ByVal lpString As String)
As Long
in a VBA function gives lstrlenA("qwerty") equal to 6. Correct.

From the worksheet, lstrlenA("qwerty")=1. Wrong

Very fine example. I will reuse it for further examination regarding
this problem.

So I would guess you have to give VBA chance to create a variable so it can
pass the string correctly. i.e. create a dummy VBA function to call each of
your DLL functions that invlove strings.

For now, this will be the way I'll do it.

Regards,
Matthias

 

[mkluwe]

I just took the first few steps in writing a little DLL that should
be called from Excel/VBA, and I stumbled upon some wierd behaviour.

The function in my DLL is declared as

int __stdcall foo( const char *t );

and is used in VBA via

Declare Function foo Lib "C:\foo\foo.dll" (ByVal t As String) As Long

I noticed that foo didn't work internally as I expected and added
writing *t to a file on each call of foo as a debugging measure.
According to this output, only the first character of String t seems
to be passed to foo when called directly from Excel (putting
=foo("xyz") in a cell).

Let me answer my own post:

Excel passes the string as some kind of wide-character. Using
const wchar_t *t in my function works perfectly.

Regards,
Matthias

 

[NickHK]

Now you mention that, changing the declare of lstrlenA to the Wide version;
Public Declare Function lstrlenW Lib "kernel32" (ByVal lpString As String)
As Long

now works fine the worksheet also.

IIRC normally when calling window function from VB there is Unicode > ANSI
conversion. Hence the "A" versions of these function are used.
If I wanted to use the W version it would be
Public Declare Function lstrlenW Lib "kernel32" (ByVal lpString As Long) As
Long
and call it with
lstrlenW(strPtr("MyString"))

From the worksheet, seems the conversion does not occur.

NickHK

 

[mkluwe]

Hi!

Now you mention that, changing the declare of lstrlenA to the Wide version;
Public Declare Function lstrlenW Lib "kernel32" (ByVal lpString As String)
As Long

now works fine the worksheet also.

IIRC normally when calling window function from VB there is Unicode > ANSI
conversion. Hence the "A" versions of these function are used.
If I wanted to use the W version it would be
Public Declare Function lstrlenW Lib "kernel32" (ByVal lpString As Long) As
Long
and call it with
lstrlenW(strPtr("MyString"))

From the worksheet, seems the conversion does not occur.

Indeed. I just found the following document:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnofftalk/html/Office03082001.asp

It says: "Although VBA uses Unicode internally, it converts all strings
to ANSI strings before calling a function in a DLL". Thus, both Excel
and VBA use Unicode internally, but only VBA insists on conversion.

Sad situation -- I still have no better option than wrapping functions
in VBA, otherwise my DLL would have to provide two different functions
for each operation.

Regards,
Matthias

 

[NickHK]

Hi!



Indeed. I just found the following document:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnofftalk/html/Office03082001.asp

It says: "Although VBA uses Unicode internally, it converts all strings
to ANSI strings before calling a function in a DLL". Thus, both Excel
and VBA use Unicode internally, but only VBA insists on conversion.

Sad situation -- I still have no better option than wrapping functions
in VBA, otherwise my DLL would have to provide two different functions
for each operation.

Regards,
Matthias

Yes, given that one converts and other does not, it would be better to
expose your function as wrapper to the private Declares. Otherwise, you will
have to rely on the user knowing to call the W version from the worksheet
and the A version from VBA.

NickHK