Building new features and functionality on top of legacy software is a special sort of challenge, one that I’ve written about from time to time.

To be honest though, the current legacy application that I’m working with is not actually that bad. The prior technical lead had the great idea to implement a relatively generic way to execute modern . NET functionality from the legacy VB6 code thanks to the magic of COM, so you can still work with a language that doesn’t make you sad on a day to day basis. Its a pretty basic eventing system (i.e. both sides can raise events that are handled by the other side), but its effective enough.

Everything gets a little bit tricksy when windowing and modal dialogs are involved though.

One Thing At A Time

The legacy application is basically a multiple document interface (MDI) experience, where the user is free to open a bunch of different entities and screens at the same time. Following this approach for new functionality adds a bunch of inherent complexity though, in that the user might edit an entity that is currently being displayed elsewhere (maybe in a list), requiring some sort of common, global channel for saying “hey, I’ve updated entity X, please react accordingly”.

This kind of works in the legacy code (VB6), because it just runs global refresh functions and changes form controls whenever it feels like it.

When the .NET code gets involved though, it gets very difficult to maintain both worlds in the same way, so we’ve to isolating all the new features from the legacy stuff, primarily through modal dialogs. That is, the user is unable to access the rest of the application when the .NET feature is running.

To be honest, I was pretty surprised that we could open up a modal form in WPF from an event handler started in VB6, but I think it worked because both VB6 and .NET shared a common UI thread, and the modality of a form is handled at a low level common to both technologies (i.e. win32 or something).

We paid a price from a user experience point of view of course, but we mostly worked around it by making sure that the user had all of the information they needed to make a decision on any screen in the .NET functionality, so they never needed to refer back to the legacy stuff.

Then we did a new thing and it all came crashing down.

Unforgettable Legacy

Up until fairly recently, the communication channel between VB6 and .NET was mostly one way. VB6 would raise X event, .NET would handle it by opening up a modal window or by executing some code that then returned a response. If there was any communication that needed to go back to VB6 from the .NET, it always happened after the modal window was already closed.

This approach worked fine until we needed to execute some legacy functionality as part of a workflow in .NET, while still having the .NET window be displayed in a modal fashion.

The idea was simple enough.

  • Use the .NET functionality to identify the series of actions that needed to be completed
  • In the background, iterate through that list of actions and raise an event to be handled by the VB6 to do the actual work
  • This event would be synchronous, in that the .NET would wait for the VB6 to finish its work and respond before moving on to the next item
  • Once all actions are completed, present a summary to the user in .NET

We’d actually used a similar approach for a different feature in the past, and while we had to refactor some of the VB6 to make the functionality available in a way that made sense, it worked okay.

This time the legacy functionality we were interested in was already available as a function on a static class, so easily callable. I mean, it was a poorly written function dependent on some static state, so it wasn’t a complete walk in the part, but we didn’t need to do any high-risk refactoring or anything.

Once we wrote the functionality though, two problems became immediately obvious:

  1. The legacy functionality could popup dialogs, asking the user questions relevant to the operation. This was actually kind of good, as one of the main reasons we didn’t want to reimplement was because we couldn’t be sure we would capture all the edge cases so using the existing functionality guaranteed that we would, because it was already doing it (and had been for years). These cases were rare, so while they were a little disconcerting, they were acceptable.
  2. Sometimes executing the legacy functionality would murder the modal-ness of the .NET window, which led to all sorts of crazy side effects. This seemed to happen mostly when the underlying VB6 context was changed in such a way by the operation that it would historically have required a refresh. When it happened, the .NET window would drop behind the main application window, and the main window would be fully interactable, including opening additional windows (which would explode if they too were modal). There did not seem to be a way to get the original .NET window back into focus either. I suspect that there were a number of Application.DoEvents calls secreted throughout the byzantine labyrinth of code that were forcing screen redraws, but we couldn’t easily prove it. It was definitely broken though.

The first problem wasn’t all that bad, even if it wasn’t great for a semi-automated process.

The second one was a deal-breaker.

Freedom! Horrible Terrifying Freedom!

We tried a few things to “fix” the whole modal window problem, including:

  • Trying to restore the modal-ness of the window once it had broken. This didn’t work at all, because the window was still modal somehow, and technically we’d lost the thread context from the initial .ShowDialog call (which may or may not have still been blocked, it was hard to tell). In fact, other windows in the application that required modality would explode if you tried to use them, with an error similar to “cannot display a modal dialog when there is already one in effect”.
  • Trying to locate and fix the root reason why the modal-ness was being removed. This was something of a fools errand as I mentioned above, as the code was ridiculously labyrinthian and it was impossible to tell what was actually causing the behaviour. Also, it would involve simultaneously debugging both VB6 and .NET, which is somewhat challenging.
  • Forcing the .NET window to be “always on top” while the operation was happening, to at least prevent it from disappearing. This somewhat worked, but required us to use raw Win32 windowing calls, and the window was still completely broken after the operation was finished. Also, it would be confusing to make the window always on top all the time, while leaving the ability to click on the parts of the parent window that were visible.

In the end, we went with just making the .NET window non-modal and dealing with the ramifications. With the structures we’d put into place, we were able to refresh the content of the .NET window whenever it gained focus (to prevent it displaying incorrect data due to changes in the underlying application), and our refreshes were quick due to performance optimization, so that wasn’t a major problem anymore.

It was still challenging though, as sitting a WPF Dispatcher on top of the main VB6 UI thread (well, the only VB6 thread) and expecting them both to work at the same time was just asking too much. We had to create a brand new thread just for the WPF functionality, and inject a TaskScheduler initialized on the VB6 thread for scheduling the events that get pushed back into VB6.


Its challenging edge cases like this whole adventure that make working with legacy code time consuming in weird and unexpected ways. If we had of just stuck to pure .NET functionality, we wouldn’t have run into any of these problems, but we would have paid a different price in reimplementing functionality that already exists, both in terms of development time, and in terms of risk (in that we don’t full understand all of the things the current functionality does).

I think we made the right decision, in that the actual program functionality is the same as its always been (doing whatever it does), and we instead paid a technical price in order to get it to work well, as opposed to forcing the user to accept a sub-par feature.

Its still not immediately clear to me how the VB6 and .NET functionality actually works together at all (with the application windowing, threading and various message pumps and loops), but it does work, so at least we have that.

I do look forward to the day when we can lay this application to rest though, giving it the peace it deserves after many years of hard service.

Yes, I’ve personified the application in order to empathise with it.