What are they and where are they?
First a little background. Many
of the functions,types, objects and commands you use in VBA come from
the VBA library. It is in this library that the code exists to manipulate
the values you supply and return the answers or actions you expect. For
example the Date property returns the current machine date.
Just out of interest...The Date can be set using
Date = "05/03/02"
and it will change the machines date,
Access seeds three library references to
get up and running.
3) DAO or ADO
Each of these libraries supply or export
a variety of functions, objects etc. For instance the Access library exports
DoCmd and DLookup, DAO or ADO exports Database and Recordset data types
(objects) to name a few of dozens.
Where are these libraries?
The libraries are files on the
hard disk. Usually dll files but may have other extensions like .olb or
.tlb. An example is 'C:\Program Files\Common Files\Microsoft Shared\DAO\dao360.dll'.
They can also be comprised of multiple files.
Why have all these separate libraries and files?
By using separating an functionality
into libraries it is possible to create a single library and let multiple
applications use it. For example: Excel, Outlook and Word all use the
VBA library. This allows you to use the same language to program
these different applications. If there is a problem with the library then
the vendor can distribute the fixed library without having to distribute
the whole application again.
In fact the complete set of VBA libraries, including the IDE (code window)
can be licensed from Microsoft. The VBA SDK allow you add VBA programmability
to any application you may write. ( although the license may seriously
max out your Visa card).
So what has all this to do with references?
The idea behind the references collection is to have a place to store
a list of the type libraries that your application will use.
When you first create a blank database, Access creates a reference collection
that includes the three libraries mentioned above.
As you gain experience with Access you may want to use other libraries
to enhance your application. The Office library will let you manipulate
tool bars or that little Clippy we all love, or perhaps you
want to use the SQLDMO library to manipulate a SQL Server database, or
even the Excel library to steal some of the nice functions
available from it.
The most efficient way to use these libraries is to set a reference to
them. When access accepts the reference it stores information about the
library in your database.
There is another benefit to giving Access this list. Type libraries dont
only contain code, they also contain the datatypes, argument lists and
names of the variables, functions etc. All the descriptive information
you see in the object browser comes from the type library. With this list,
Access can instruct VBA to compile your code, checking with the type library
that you have used the objects and functions correctly, into an efficient
stream of PCode because It knows what libraries are used and how best
to call them in advance. (This is called early binding). You can reference
many libraries from a variety of sources. Third party controls (ocx files)
have type libraries built in (usually). When you add a third party control
to a form Access automatically adds the reference to the list.
What happens if I accidentally delete a dll file that is a reference?
This is where it gets annoying. If
this happens and you open your database there is a good chance that the
code will stop somewhere with the message Cant
find project or library. It unfortunately usually
stops and points to Date or some other function / command that has nothing
to do with the missing library. (At least you know something is wrong)
What you must then do is open the references from a code window. Its
usually under the tools menu.
Notice the missing reference! (5th down)
By high-lighting the reference with
the mouse you can see where the library file is. Now comes the fun part.
This broken reference could result from the file being missing. So go
to the location shown and see if its there. If not..well where the bleep
did it go! You will have to search around the hard drive to see if you
can find it or re-install the library (or post on Microsoft.public.access.forms
whatever..news group). But there is another evil that can befall your
library. It is not hard to imagine that another application installed
recently also uses this library. But it actually uses an older version
that doesn't support the stuff that your version did. Thus all the precompiled
code magic is wasted because some of the functions required and optimised
by your application are not in the new library. VBA immediately decompiles
your code as it knows something is very wrong because it loads and checks
the libraries prior to running your code / application. This scenario
can be quite a problem as the path shown on the reference window will
point to the new library location. Your only hope here is to search your
hard drive using 'Find files or folders' for the file and see if the original
is elsewhere. If you find multiple files, look at the dates and sizes.
The culprit may be dated earlier. By selecting the file in the search
window and pressing [alt]+[enter] the properties box will come up. Here
you can see the version number.
If my original version is still on the drive, how come the other version
is being used?
Type libraries are registered in the
operating system registry. One of the entries is the location (path) to
the 'current' version of the file. Installation software (bad ones) will
not check versions and put its version somewhere on the hard drive and
register the new library as the current in the registry.
It is not unknown for bad installation software to actually replace the
later version with an old one and thus your original is gone!
Can I re-point the registry
to the version that works.
Yes. You will notice on the tools menu another option call ActiveX controls.
Press the Register button. This will allow you to browse the drive, find
your library and re-register it. You may have to change the extension
selector in the browse box to the type you need. i.e.: from *.ocx to *.dll
or *..tlb. Amazing it's so handy to the references menu <g> You
can also re-register your library by using the 'Run' button on the start
button menu in Windows. Type 'regsvr32 c:\1\burner.ocx' (without the quotes
and using the name and path of your library)
Sometimes after registering the library
again, you must un-tick the box, hop out of references and then hop back
in again, find and re-tick it.
Recompile your code - hopefully you're back in business
For further information on this article contact
peter walker firstname.lastname@example.org
Other information relevant to this topic is DLL Hell