Bad DLL Calling Convention Error in Microsoft Access Databases Fixed by Decompiling

Bad DLL Calling Convention Error

Have you seen Error 49, Bad DLL calling convention, when running what appears to be your perfectly, well written code?

When this error occurs, it's usually after one of your procedures finishes and returns to its calling procedure. However, the message makes no sense because you haven't called a DLL. Debugging and stepping through your code drives you crazy because everything is working fine, yet the error keeps popping up.

It's driving you crazy and enough for you to throw away Access! What's going on?

Queries with VBA Functions Suddenly Fail

Similarly, do your queries that have VBA code (such as Left, Mid, or UCase) start failing? How can something so simple all of a sudden stop working?

Compact and Repair doesn't help

This can all be very confusing because it makes no sense. Everything looks fine. And Compact and Repair does not help.

The initial assumption is that the database is corrupt, which is partially true. People then resort to creating a new database and importing their objects into it which is time consuming and a pain. It solves the problem, but there's a much easier way.

Fortunately, there are a few options that can resolve this problem.

Are any Library References Broken?

The first thing to check is whether the library references for your module code are valid. From the VBA IDE, go under Tools, References to see the list of referenced libraries and make sure none of the checked items say "Missing" in front of them. If so, fix that and your problem should go away. Assuming that's not the cause of the problem...

The Problem Lies with the Compile State of VBA

Unfortunately, over time, the part of your Microsoft Access database that stores VBA module code can become bloated, and in some cases, trigger strange errors such as Bad DLL calling convention.

As you compile and run code, VBA leaves behind old versions of the code that are no longer valid, but it is not always removed from the database. Database Compact and Repair only addresses the Jet tables in your database, and not the code. That's why it has no impact when this error arises.

The Microsoft Access /decompile command

Fortunately, Microsoft Access has a solution with the /decompile command line option.

When MS Access is started with this and you open a database, it discards all the old compilations and leaves just your VBA source code. You'll need to compile again to get the compiled state. After compacting, your database may be significantly smaller.

To decompile your database, follow these steps.

1. From Windows Explorer, locate where your Access program is installed by searching for MSACCESS.EXE. If you have multiple versions of Access installed on your machine, you need to identify the path to the Access version you want.
2. From the Windows, Start, Run command line, type: msaccess.exe /decompile, where msaccess.exe includes the complete path. For example, the 32-bit version of Access 2016 is commonly installed here:
   C:\Program Files (x86)\Microsoft Office\root\Office16\MSACCESS.EXE /decompile
3. When Access opens, open the database you want to decompile (with trusted authority for Access 2003 or later)
4. Open any module in design mode and compile it from the menu Debug, Compile.., then File, Save.
5. Go back to the database and Compact it. The location of the Compact command varies by Access version.

The database size should be reduced and the strange errors related to the VBA code should be gone.

To simplify doing this in the future (and you'll likely need this again), create a shortcut on your Windows desktop pointing to where your copy of Access is located:

C:\Program Files (x86)\Microsoft Office\root\Office16\MSACCESS.EXE /decompile

Then, whenever your database is acting oddly, you can easily launch the decompile version of Access to open your database and decompile it. In a few minutes, you're back up and running with a smaller database.

Good luck!

• Creating a Backup and Disaster Recovery Plan for Microsoft Access Database Applications
• Compact on Close command
• How the decompile command eliminates VBA related errors
  Decompile Your Microsoft Access Database to Improve Performance, Fix Corruption, and Avoid Strange Errors
• Microsoft Access Inconsistent Compile Error for a Field Reference in a Form
• All Microsoft Access Error Numbers and Descriptions
• Microsoft Access Developer Corner