ACCESS FEED: RE: [MS_AccessPros] Conditional API declaration

Kamis, 28 Agustus 2014

RE: [MS_AccessPros] Conditional API declaration

Hi Stuart

VBA7 is a built-in compiler constant which is used in a conditional compilation #If statement to determine the version of VBA which is compiling the code. If VBA7 is True, then the version is 7.x (or later, but no later version exists yet) and the new syntactic elements that were introduced with Office 2010 are understood by the compiler. In older versions, VBA7 is not defined (therefore it cannot be True) and this indicates that the programmer should not use the new features.

A conditional compilation #If statement differs from a traditional If statement in that if the compilation condition is not True, there is no requirement for syntactic correctness, while in a traditional If statement, all the code must be syntactically correct, even though it might never get to run. This code, for example, will compile OK, even though it is gibberish to the compiler:

#If False Then
This is not a comment but it makes no sense at all %$ > ???
#End If

You can also define your own compiler constants to turn bits of code on and off. For example:

#Const Debugging = True
. . .
#If Debugging Then
MsgBox "We made it to Point A"
#End If

I use them all the time to turn on and off early binding of object libraries.

Cheers,
Graham

From: MS_Access_Professionals@yahoogroups.com [mailto:MS_Access_Professionals@yahoogroups.com]
Sent: Friday, 29 August 2014 01:18
To: MS_Access_Professionals@yahoogroups.com
Subject: Re: [MS_AccessPros] Conditional API declaration

Graham, what is "VBA7" exactly? Is it a property of something?

Thanks, Stuart

On Wednesday, August 27, 2014 8:39 PM, "'Graham Mandeno' graham@mandeno.com [MS_Access_Professionals]" <MS_Access_Professionals@yahoogroups.com> wrote:

I stand humbled and corrected! It was Office 2010, not 2007, which introduced VBA7 and 64-bit. Sorry!

So maybe we're stuck with the dual declares for a bit longer :-)

Cheers,
Graham

From: MS_Access_Professionals@yahoogroups.com [mailto:MS_Access_Professionals@yahoogroups.com]
Sent: Thursday, 28 August 2014 09:55
To: MS_Access_Professionals@yahoogroups.com
Subject: RE: [MS_AccessPros] Conditional API declaration

Hi Crystal

I changed the subject :)

The main point is that code for 64-bit VBA must include the PtrSafe qualifier in the Declare statement. This is not required for 32-bit VBA, but its absence will prevent compilation on 64-bit. It is, however, understood by both bit-ness of VBA7, so should be included in all Declares for VBA7 onward. (VBA7 is currently the latest version).

However, it is not understood on versions pre-VBA7 (pre-Office 2007) and will cause a compiler error. This is why we need to include the #Else block with the old format of the declaration, so that the code will work in Access 2003 and earlier. I guess that as pre-2007 versions of Access disappear, the need for the conditional Declare will also largely disappear, but I don't see that happening soon. (Look at Tom Wickerath and Boeing, for example!)

By the way, PtrSafe doesn't actually do anything! It's just a way to make sure that the programmer has read the Terms & Conditions and at least made an attempt to ensure the code is 64-bit compatible :-)

The other main difference is in variables which are pointers (i.e. addresses of locations in memory). 32-bit code addresses memory with 32-bit pointers, which can be stored in a Long variable, but 64-bit code uses 64-bit pointers which obviously don't fit. VBA has therefore introduced a new data type, LongPtr, which is a Long in a 32-bit code base, and a new 64-bit integer data type LongLong in 64-bit code. Using Long instead of LongPtr will work fine in 32-bit, and will compile OK in 64-bit but will crash and burn at runtime because the API will be reading and writing 64-bit values where only 32 bits have been allocated for storage.

A very few API functions have versions that actually use 64-bit integers (not pointers) and for these the arguments need to be declared as LongLong. VBA does not understand LongLong in 32 bit, so there is another compiler constant Win64 which is True if the code is running in 64-bit mode. I have seen code where programmers have used #If Win64 instead of #If VBA7 for the two types of declaration. While this will work OK, it is not technically correct, because VBA7 is designed to understand both types of pointer. Note that Win64 indicates the bit-ness of the office application code, not the bit-ness of the Windows operating system it is running on (as is stated in some documentation).

Hope this clarifies things a little :-)

Cheers,
Graham