Tuesday, November 20, 2007

[Ada (ObjectAda 7.2.2)] Creating VB6 DLLs

Creating DLLs for use by VB6 seems to be somewhat of a dark art. From what Roger Pearse, in his The Noddy Guide to using ADA code with Visual Basic says, it doesn't appear to be something easily achieved.

So far I've achieved it, to a certain extent, with ObjectAda and GNAT/GPL. The code for the ObjectAda implementation is below. I'll post the GNAT/GPL stuff in the next couple of days, God willing.

BO_DLib.adb, BO_D.adb and BO_D.ads are all based, albeit rather loosely, on sample files in winapi\samples\petzold\ch19\edrlib\

First up, BO_DLib.adb

Next, the spec file, BO_D.ads

There are six routines:
  • procedure Bob1 calls dobox to show a messagebox.
  • function Bob2 returns a long value, 1.
  • function Bob3 receives a long and adds 21 to it.
  • procedure Bob4 receives a long pointer to a string and passes it to dobox for display.
    I can pass Unicode to it and have it rendered appropriately.
  • function Bob5 receives a long pointer to a string and returns a long being the length of the string.
  • function Bob6 receives a long pointer to a string and catenates a string to it.
    "Danger, Will Robinson!" ... this is not how to do it and VB complains bitterly.
Okay, there are seven routines, but dobox isn't exported. It takes a pointer to a wide string and passes it to Win32.Winuser.MessageBoxW. I got dobox from a sample file as well.

And, finally, BO_D.adb, where the definitions given in BO_D are fleshed out.

Now the VB code. Take note of a few things:
  • I declared the functions and procedures as DLL_Stdcall in BO_D.ads so the routines come in decorated, thus the use of Alias in the Declare line. I used Microsoft's Dependency Walker to discover to what extent the names had been mangled.
  • A reference to SCRRUN.DLL was added to the project, making possible the use of the FileSystemObject.
  • Bob6 barges past the limit set for the string by VB, so things get a bit unstable. I'm still working through how to pass strings back.

By the way, if you think my code sucks, say so. I'm a total newb when it comes to Ada. Critiques are always welcome. King Solomon said, "The wise person accepts instructions." and also "Better is open rebuke than hidden love."

© Copyright Bruce M. Axtens., 2007

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