Unity 2018.2.1 and VS2015 CE - can not attach debugger

We're using Unity 2018.2.1, Visual Studio 2015 Community Update 3 and Visual Studio Tools for Unity 3.7.0.1.

We've also added an Editor script per @sailro's post in this thread.

Our scripts open in VS2015 and we can edit and work on them without issue. Unfortunately, we can't attach the debugger, with this error:

 

Any particular reason you're using 2015 and not 2017? Or are you sure it's not 2017? I didn't realize 2015 had a CE.

 

Yes. A project I'm working on requires 2015 and my preference is to not have both installed.

If that's what I have to do I will, but VS2015 is still supposed to work.

It sure does.

 

Ahh it does. It's an interesting error... looks like it's trying to load referenced assemblies from both the Mono install and the .NET Framework install. Honestly, I would go ahead and install 2017 side by side (I've run it side by side as well as running the Previews side by side). It's a pain to have to install and maintain another version but 2017 is miles faster and more stable than 2015.

 

Yeah, I used to have side-by-side installs of '15 and '17, until one of them got corrupted and wouldn't build, repair, uninstall or reinstall. Troubleshooting that, and then ultimately reinstalling my whole system to fix it, cost me far more productivity than I'd have liked. Hence my very strong preference for keeping it as simple as possible this time around.

 

Well, as a result of nothing I've deliberately done, my VS2015 debugger is working again.

 

And now, as a result of nothing I've deliberately done, VS2015 is just not integrating with Unity at all. It opens when I double click a source file from Unity. Then it attempts to load each of the Projects in the Solution and they each just get marked as "unavailable". And the debugger then of course doesn't work. (In fact, VSTU doesn't even seem to be loading as it is no longer "Attach to Unity...")

Since it's busted anyway I'm now installing VS2017 and just hoping things work out afterwards.

For what it's worth... everything was groovy until I moved away from the LTS version of Unity.

Edit: And now that VS2017 has finished installing, VS2015 with VSTU is working with Unity again. It's just intermittent without explanation.

 

I wonder if something is/was busted with the VSTU plugin. Actually, I'm not sure if the newer versions of VSTU are supported in VS2015, so it's possible there was a compatibility break with newer non-LTS versions of Unity.