Archive for July, 2013

VBA for 32 and 64 bit systems

Microsoft Access I’ve put off writing about x64 MS Office because I was a bit lazy and hadn’t build the necessary environment to test it properly.

In my day-to-day job -as far as Access is concerned-, I have so far targeted the 32 bit (x86) version of Office 2010 but the 64 bit version is bound to become more popular with time and some people already have to use it, even if sometimes their reasons are misguided.

When should I care?

If your code has any chance of being run on Office 2010 and later, and you want to avoid issues when one of your users or customers will have Office x64 (or just one of its components) installed, then you should make sure to use your code uses the new VBA7 constructs.

The changes to be performed only affect the parts of your code that use Win API calls or manipulate Windows control handles and memory pointers.

For a small fraction of applications, actually using Access/Excel x64 could make sense as it allows you to deal with 64 bit integers (as opposed to the standard 32 bit long). While this could be a useful feature, code that use these 64 bit integers will not compile on a 32 bit version of Office.

If you use external libraries and controls

If you are using external libraries and ActiveX controls, it’s unlikely that they will only work in Office x64. The reason is that most libraries (especially older ones) are only available in 32 bits.

For these to work under Office x64, they would have to be compiled especially for 64 bit as well.

So, the bottom line is: do not use external libraries and ActiveX components (not even the ones that usually come with Office!) unless you can either:

  • completely control the user environment an ensure it will always run Office x86 (32 bit).
  • you can compile or get 64 bit versions of these libraries and ActiveX components so you can build a version of your application that will work for all your users.

If your code targets Access 2010 and later

There are 2 changes that may affect your code: external library declarations and usage of the new pointer and very large integer types.

Library declarations:

Any declaration for external libraries (such as Win API or code that resides in DLLs), the declaration statement has changed and must use the new PtrSafe keyword to tell the compiler that the declaration should be interpreted in a 64 bit context when relevant. If you omit the keyword, your declarations will always be interpreted as being in a 32 bit context and the results may be random.

For instance, the old declaration for the SetFocus Win API function was:

    Private Declare Function WinAPISetFocus Lib "user32" _
                    Alias "SetFocus" (ByVal hwnd As Long) As Long

Now it is:

    Private Declare PtrSafe Function WinAPISetFocus Lib "user32" _
                    Alias "SetFocus" (ByVal hwnd As LongPtr) As Long

Note how the hwnd parameter is now a LongPtr instead of just a Long.

New types:

You will need to replace declarations that use long with the more appropriate type:

  • LongPtr
    Use the new LongPtr wherever you deal with declarations that relate to Windows control handles or memory locations.

  • LongLong
    Defines a 64 bit integer.
    The reason for LongLong is mainly to access some 64 bit versions of the Windows API, in particular, for math functions.
    This type only exists if you are running under a 64 bit version of Access. On 32 bit systems, it will cause a compile error. To avoid that problem where your code needs to run on both 32/64 bit platforms, you can use the new conditional constant Win64:

        #If WIN64 Then
            Dim largeInt As LongLong  ' Declares a 64 bit integer
        #Else
            Dim largeInt As Long      ' Declares a standard 32 bit integer
        #End If

The compiler will only see and compile one code path and will ignore the other, so doing this does not affect the speed of your application.

If your code must also work in Access 2007 and earlier

Older versions of Access will not be able to compile the new PtrSafe, LongPtr and LongLong keywords introduced by VBA7. If your code must also work in older versions of Access, you must either create 2 versions of your application (not very convenient), or simply use the new conditional constant VBA7 to separate blocks of code that will run on Office 2010 and later from blocks of code that will run on Office 2007 and ealier.

The new VBA7 conditional constant will evaluate to True if it is running under Office 2010 and later.

So now you can create variations of your code that will work everywhere:

    #If VBA7 Then
      Private Declare PtrSafe Function WinAPISetFocus Lib "user32" _
                      Alias "SetFocus" (ByVal hwnd As LongPtr) As Long
    #Else
      Private Declare Function WinAPISetFocus Lib "user32" _
                      Alias "SetFocus" (ByVal hwnd As Long) As Long
    #End If

Adapting old code

When going through old code, you can’t just add PtrSafe to all API declarations and do a search to replace Long into LongPtr. That would be the surest way to get everything wrong.

Instead, you must be very careful about the intent of the code and understand what your variables are manipulating: is that long really just a number or is it mean to be a memory location or a control handle ?

To help you, at least as far as the most common Win API declarations are concerned, Microsoft has made available a text file with hundreds of such declarations compatible for Office x64: Win32API_PtrSafe with 64-bit Support

If you need to use a WinAPI call that isn’t in the list, you will need to have a look at its actual definition and, based on the type of data being passed around, decide if you need to use a simple Long or a LongPtr.

For instance, I had to use the MonitorFromWindow API function. Its C++ definition is as follow:

    HMONITOR MonitorFromWindow(
      _In_  HWND hwnd,
      _In_  DWORD dwFlags
    );

The definition tells me:

  • That the function is taking a Handle (basically a pointer) to a Window and some flags
  • and that the function returns a Handle to the display monitor (basically a pointer).

So the API definition that will work on all versions of Access is:

    #If VBA7 Then
        ' Works in 32/64 bits of Office 2010 and later
        Declare PtrSafe Function MonitorFromWindow Lib "user32" _
                                 (ByVal hwnd As LongPtr, ByVal dwFlags As Long) As LongPtr
    #Else
        ' Works in Office 2007 and earlier
        Declare Function MonitorFromWindow Lib "user32" _
                         (ByVal hwnd As Long, ByVal dwFlags As Long) As Long
    #End If

When using the external function, you also need to be careful in the code and use the VBA7 conditional to separate the parts where you access structure pointers:

    Private Sub Redimension()
        #If VBA7 Then
            Dim hMonitor    As LongPtr
        #Else
            Dim hMonitor    As Long
        #End If
        hMonitor = MonitorFromWindow(Application.hWndAccessApp, &H0&)
        ... do stuff with hMonitor ...
    End Sub

Make sure you save, re-compile and test your project as you go along. Also make frequent backups of your application as a wrong declaration could crash and corrupt it.
It may even be possible to crash the whole machine, although I have not experienced that so far.

Conclusion

Updating old code to make it work properly on all versions of Access is not difficult. It is usually fairly easy, although it can be a bit cumbersome at times.

The trick is to know what your code really needs to target:

  • Office 2010 and later, the present and the future, in which case everything is straightforward and you use the new VBA7 keywords PtrSafe, LongPtr and LongLong where appropriate.
  • All versions of Access, in which case you will have sometimes to separate and maintain 2 versions of your code using the VBA7 conditional.

References

4 comments July 7th, 2013

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