Original message:

Hi List,

The ArcView help pages "Writing your own DLL" mention that since VB5 does
not support "exported" functions, it cannot be used to create dlls that
ArcView can read.

However, this should be possible in VB6.  Is that correct?

If so, does anyone have the code of a simple dll created in VB and accessed
by ArcView?  I can use other dlls in my Avenue code, but I'm not sure how
to declare an exported function in a VB ActiveX dll.  Even something as
simple as the math.dll (written in C) described in help file would be
great.  All that dll does is add two numbers together.  AV passes the two
numbers to the dll, and the dll sends the result back to ArcView.

Thanks for your help.

Bryan

Summary:

The simple answer is that you can't.  Scott Engel
(scotte@newcenturysoftware.com), Michael Hickey (MHickey@co.tulare.ca.us),
Rineer, James (JRineer@dewberry.com), Rautenberg, Volker
(V.Rautenberg@pro-crm.de), Fene, Michael (MICHAEL.FENE@saic.com), and Ro
bert van Waasbergen (robertvw@aeds.com) all wrote to confirm that.

Bruce Cotton (Bruce.Cotton@agso.gov.au) sent a Word file documenting an
error in the AV help files for writing a DLL in C++.  E-mail me if you want
a copy of that.

Jorg Thomsen (jthomsen@mapmedia.de) sent the most detailed reply suggesting
a couple workarounds.  Here is Jorg's answer which was also a SUM to this
list some time ago:

Hi bryan,

   You can't.

The reason is:
   Visual Basic only creates ActiveX DLLs.  This means that you can only
use
   these DLLs in an OLE-enabled environment, of which ArcView is not.

   Visual Basic compiles an ActiveX DLL, which is nothing like a true dll
   containing exported functions. There is no easy way within Visual Basic
   to export a function

   I recently ran into a similar situation where I had to integrate ArcView
   with another system based entirely on VB DLLs that had been developed
over
   the course of several years.  After posing your question to every ESRI
   person I could at the user conference, I came away with a simple answer-
you
   can't do it directly.  C++ DLLs work fine, but not VB

   Yup, VB dll's don't work with ArcView.  The reason is b/c ArcView can on
   call "Old School" DLL's that export their functions.  VB only creates
COM
   dll's that do not export their functions.  COM uses GUID's and other
   Microsoft voodo to make the function call.

   AV does not work with ActiveX DLLs at all (VB or otherwise).  This is
   according to ESRI.

BUT there are some work-arounds:
   If you've already got the DLLs coded and compiled, one trick would be
use
   Visual C++ to write a wrapper around the VB DLL.  This wrapper would be
a
   standard DLL whose exposed functions pass through to the VB DLL.  Since
   Visual C++ can use ActiveX components, this should theoretically work.
 I
   gues it could alos work in any environment that supports ActiveX.

   Purchase PowerBasic - a Basic dll compiler that will produce a true dll
   with exported functions. For about $300 US you can get both the compiler
and
   the dialog designer. This is probably the easiest method with the
smallest
   amount of learning to do.

   Supposedly Sheridan Software has a control that will allow you to export
   functions in a Visual Basic dll. I have never been able to get this to
work,
   but you might consider it as a last resort.

   My solution was to create a VB executable that reads a "transaction
file"
that
   avenue creates, performs the appropriate action, then deletes the file.
I didn't
   want to use DDE because I had to guarantee it would work.  The exe is
called
   from avenue with a "system.execute" request.  Actually, in my
application, the
   exe is started only once and sits listening for transaction files to
   process, then it quits when ArcView quits.  Kind of a hokey solution,
but
it
   works and was easier than trying to get our VB guys to switch to C++.

   Es gibt einen Freeware-Basic-Compiler namens XBasic, mit dem man echte
   DLLs erstellen kann (hab's selber noch nicht getestet). Zu finden ist
   er bei
   http://www.maxreason.com/software/xbasic/xbasic.html
   oder bei
   http://ntfreeware.zdv.uni-tuebingen.de/NTSOFT/default.htm

O.K., that's all. I Think we'll try the xBasic-Solution and if that does
not
work we will
create an executable.

Thanks again to:
   Nick Seigal, Criterion Planners/Engineers
   Thomas Sakowski
   Don Browning,  GIS Analyst/Programmer at GeoFocus, Inc.
   Timothy E. Johnson, Maricopa County Elections Department
   James Marlow
   Bill Dollins
   mvh matts, Soil and Water, Helsingfors, Finland
   Mike

Jorg Thomsen
MapMedia - Kartographie und raumbezogene Informationssysteme
MapMedia - Cartography and spatial Informationsystems

Am Borsigturm 42, D-13507 Berlin
Tel.: +49 (030) 43 03 21 00
Fax: +49 (030) 43 03 21 01
eMail: jthomsen@mapmedia.de
internet: www.mapmedia.de


Thank you to everyone for the answers.
__________________________________________

Bryan Keith
GIS Specialist
Geomega, Inc.
Boulder, CO, USA
bryan@geomega.com