How to detect windows session lock/unlock in VBA

There are (at least) 3 solutions to detect when a user locks his/her workstation on Windows and react on the event.

I’m trying not to use C (or any other language/tool) to write a DLL, although that would be also easily possible.

The windows API function that allows us to be notified when a user session is locked or unlocked (among other notifications) is the WTSRegisterSessionNotification() function.

Solution 1: The Classic Visual Basic 5 solution

We generate a simple 32bits standalone EXE application:

Get the binaries:

SessionLockDetector.exe (13.5 K, MD5: 57f23cb31c6f9ccd2a35c52384b90ff5)
If you need the VB5 runtime DLL (should already be present on XP up), download it here, unzip it and drop it in the same directory as the SessionLockDetector.EXE (.zip MD5: 7f32b01c04b0208ac917d7e266eb6709, extracted .dll MD5: eac679185ad621eeace9b6b286372f27).

Get the source code: SessionLockDetector (standalone) Visual Basic 5 project.

This task in quite easy in VB5 and it is also nicely stable. We just have to subclass the Form to be able to receive and handle the windows messages sent by the system.

Solution 2: The (very) bad solution

You might be tempted, as I did, to simply jump into a new Access database and apply the same technique. Just add the standard and class modules from the vb5 project, and subclass an Access Form like we did for the VB5 Form.

But Access is a different beast than vb5.

Subclassing the Form (in VBA) will make Access quite irresponsive at some point, because the subclassed form will throw a lot of messages in the WindowProc hooked into the (single threaded) VBA interpreter. Access will enter an almost unbreakable loop trying to keep up handling the event cascade while also trying to do its own work (you will see the VBA IDE window title bar endlessly update its text, if you try to run the sample database; but don’t do it please, take my word for it). At this point, even Windows itself (the desktop explorer process) will become frighteningly irresponsive. So take at look at this database code if you’re curious, but, once again, DO NOT EXECUTE / OPEN the “frmMain” form it contains (open in design mode only, don’t double-click it).

You’ve been warned. Download the very bad database that you shouldn’t open, at your own risk.

Solution 3: The hybrid solution

This is where we build an ActiveX EXE server with the same VB5 code that we used in the first solution, but we wrap the functionality inside an automation class. This will be the “LockUnlockDetector” class (which fully qualified with the name of the ActiveX server is “SessionLockDetector.LockUnlockDetector”), that will throw just two events, one for when the session was locked (OnSessionLock), and the other when it unlocks (OnSessionUnlock).

Building the VB5 ActiveX EXE server

Here’s the code of the LockUnlockDetector class module (VB5, 32bits):

By the way, according to the coding guidelines published here, we’re creating a class which will be made available outside of the VB project (that’s the goal), so we do not use any prefix before the class name (no “C” preceding the name).

To create an ActiveX EXE server, we set the properties of the project like that:

Now every object instance of class LockUnlockDetector will run in its own thread, and as this thread is in the EXE, it runs in the background and isolates the host (Access) from any problem occurring in the EXE.

Best of all, even if the EXE is 32bits, thanks to the beautiful architecture of COM (ie. ActiveX), cross platform calls are transparently marshalled and a 64bits Access VBA host can also use these 32bits automation classes.

The only disadvantage of this solution, is that we have to register the EXE server on the target machine.
In order to do that, we open an console window in administrator mode, navigate to the directory where the compiled EXE server is, and run it. This will silently write the required windows registry entries, and we’ll see the automation server appear in the Project References dialog of the VBA IDE in Access.

To use the detector class in Access, we then add the reference to our project:

Then we create a form (named frmMain here), and we just need this code:

Then we can respond to Lock/Unlock events as we wish:

In the provided sample Access database I added an history table to store the events, that’s what the AddHistoryRecord function does:


AxSessionLockDetector.EXE – ActiveX EXE server binary (if you’re missing MSVBVM50.dll see the above download link in solution 1).
SessionLockDetector – VB5 ActiveX EXE server project and source code.
SessionLockDetector.accdb sample database using the EXE server (remember to register the SessionLockDetector ActiveX server (AxSessionLockDetector.exe) binary by running it once with administrator privileges).


MIT Licence

Copyright © 2019, Francesco Foti,
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