I had to deal with the same problem of returning strings to VB recently. I
don't know if this is the best way, but it worked for me. Seems to me that
any way you slice it you have to get VB involved since it has it's own
memory manager for strings (as does Delphi), so I defined a VB module that
implemented a string-assignment "server" Sub :
sub VBAssignString( byVal src as String, dst as String )
dst = src
end sub
Note that "src" is "byVal" and "dst" is "Byref" The VB app must provide the
server to your DLL, say at startup:
InstallServer addressOf VBAssignString
Your Delphi DLL will use this procedure pointer to convert form PChar.
Finally, provide a VB-friendly interface, say, VbCallEngine. The VB module
can alias the name if you want VB apps to see "CallEngine".
Note:
1) VB will be making a copy of your returned strings, so you'll need to free
them in VbCallEngine.
2) VB passes a pointer to a pointer to a BSTR type for ByRef strings. You'll
need to declare them appropriately when receiving them, and when passing
"dst" to the server. A BSTR is a 32-bit length-preceded, null-terminated
string, and the pointer is to the string, not the length (similar to Delphi
strings).
3) You must define the length of "src" - VB seems to check this even though
the string is null-terminated. This means you have to copy the returned
string to a buffer area, like
type
TBstr = record
len: Integer;
str: array[0..MAXLEN] of Char;
end;
and pass @Bstr.str as "src".
BTW, my DLL wasn't in Delphi, so I had to take the same approach for Delphi
clients (i.e., created a delphi unit containing a delphi string-assignment
server). The Delphi version was a little more elegant because the server
could be installed automatically by the unit without the app knowing the
details.
If there's an easier way, I'd like to hear about it.
HTH,
Jim Green
Quote
Michael A. Hess wrote:
> Greetings,
> I have written a Delphi 3.0 DLL that I would like to be able to call
> from Visual Basic. The DLL works when used with Delphi but I don't know
> exactly how to write the Declare statement in Visual Basic or how to
> pass the parameters.
> The exported Delphi Function is:
> function CallEngine(dataBase : PChar; searchString : PChar;
> var hitList : PChar; var nameList : PChar) : integer; stdcall;
> The variables dataBase and searchString are passed into the DLL for
> processing. I assume they are declared "ByVal database as String".
> However, the variables hitList and nameList are strings in Delphi that
> are dynamically allocated within the DLL and I need to pass those
> strings BACK to Visual Basic. How do I declare them and how do I DIM the
> variables to pass to them.
> TIA
> --
> ----------------------------------------------------
> Michael A. Hess - Programming my first best destiny!
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