Visual Basic / Paradox Engine Demo

Written by Steve Jackson
9152 Brabham Drive
Huntington Beach, CA 92646

Source code free to all.

Comments, questions are welcome. If you know of any
ways I can earn a little extra income to purchase a
faster computer (and with more memory of course) that
would be welcome too.

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A Visual Basic application called "Visual Video" that
uses Paradox Engine for database access. It is NOT
meant to be a viable video tape rental system - just a
little sample of calling the Paradox Engine DLL from
Windows VB. No exe file is included to keep zip file
small. Also, the DLL files are not included.
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Assumptions:
You have a working knowledge of Paradox Engine
You have the Paradox Engine DLL - PXENGWIN.DLL
You have Visual Basic and its DLL

Suggestion:
Load this in a separate directory.
Put this name in db_dir in GLOBAL.BAS
(so the app will know where tables are)

Files included:
VVDEMO.MAK Project make file
PXMODULE.BAS Engine DLL definitions
and calls to engine functions
VVDBMOD.BAS Customer database routines
VVDBMOD2.BAS Item database routines
VVGLBL.BAS Global defines -
but no engine stuff
*.FRM Various forms
*.DB and *.PX Customer and Item databases
VVDEMO.TXT This file


BACKGROUND

The Paradox Engine comes with a DLL library with all the
engine functions except for PxNetInit() which is handled
a little differently in Windows. Visual Basic can use
most of these easily (more on this in a moment) by
defining them as DLL declarations. The important thing
is to define the C argument types in equivalent Visual
Basic types, and it is VERY important to use ByVal
properly to define variables passed by reference. Also,
strings passed to functions for update should be
defined as fixed length (see PxGetAlpha). C does not
understand VB's variable length strings and will
promptly crash your app if you get it wrong.

Most of the engine functions return an int as an error
code, and this works quite well in VB. I called a
generic error routine after most calls that traps show-
stopper errors from normal expected errors like not-
found, etc. At least one of the functions returns a
pointer to a string - PxErrMsg, and I have not yet
worked out how to use that in VB so for right now my
error routine just shows the error number. Some sharp
programmer could probably get some windows memory and
grab a pointer to it, but those kind of API calls are
beyond the scope of this little tutorial.

STRUCTURE OF VVDEMO

I tried to put all the Paradox Engine declarations in
one BAS file to isolate it from the rest of the app. I
also have one BAS file for each database with various
functions for Startup, Get, Add, Update, Delete, etc.
etc. This results in a two layer approach: The form
objects call the generic functions in VVDBMODx.BAS which
in turn call the Paradox specific functions in
PXMODULE.BAS. The reason for this is to make it easier
to change the Paradox specific routines without affecting
the application, and to keep your global file from
getting cluttered up. You may love it or hate it, but
at least I had a reason for it!

By the way, if you create module files as plain ASCII
before editing them in VB, they will remain ASCII files
that you can use with your favorite editor. If you
start a new module file in VB, it saves it in a weird
tokenized form. (After all, it is basic you know)

NETWORK INIT

In the dos version, you can initialize to whatever kind
of network you want with PxNetInit. In windows, you
must always init with PxWinInit, and it picks up the
desired network from settings in WIN.INI. I came up
with a way to change this at run time in a C program
like this: I put up a dialog box with different network
choices. When the user clicks OK, I wrote to WIN.INI
with the WriteProfileString() API call to change the
settings, stopped the engine with PxExit(), and restarted
with PxWinInit(). This picked up the network change (or
userid or net drive) without stopping the app. I have
not tried it yet in VB, but it worked fine in C.

FINAL WORDS

Thanks to my friend John Jaster who gave me most (but
not all) of the Paradox DLL declarations. I added some,
and corrected a few. They are still not complete, but I
leave that to the next person in this chain. Or if
anyone reading this wants me to finish it up, let me
know, and I'll find the time!