The subject line says it all.

Everything was working fine, then, out of the blue, this afternoon I have no more Intellisense in VS2012 Pro. I've tried resetting the Import and Export settings back to the default Visual C# Development Settings, but that did squat. Also my Solution Explorer pane doesn't explore my solution (no more file hierarchy).

What gives? I'm no IDE expert so I don't fiddle with the settings much. I just know this "ain't what it used to be" at all. I've Googled, I've failed to fix it.

Try deleting/renaming the SOLUTIONNAME.suo file.

Other than that, http://stackoverflow.com/questions/13375961/visual-studio-2012-intellisense-sometimes-disappearing-broken seems to have a few suggestions (1 of which you mentioned, but several others you didn't).

Thanks. I've managed to work through all of those suggestion earlier today with no luck. I'm wondering if this has something to do with Unity and the Mono version of .NET -- I was working with a newer version of .NET (4.5.something) yesterday on an unrelated non-Unity project.

Anyone think I'm on the right track with that notion?

These have all been promising suggestions, but none of them have eliminated the problem. I've deleted .SUO and .SLN files (ankhd, I've not tracked down an .NCB file, though I don't know which "app" you are suggesting).

I did a repair installation of VS2012 -- which I've taken to calling the Mayan Doomsday IDE -- and then just created a new console application. Worked fine -- proper Intellisense, proper Solution Explorer hierarchy of files. Made a "Hello World" that compiled and ran perfectly.

Loaded my Unity project, same damn problem as before.

Loaded an old project, a little thing that I was using to test AI scripts, everything in VS2012 is acting Intellisensibly.

As soon as I sort out my morning errands I'm going to create another new Unity project and see what this one does. I can't say I enjoy the bi-monthly bouts of upgrade issues, random cock-ups, and general incompatibility with modern .NET that Unity Technologies has been providing. Always, always, always blame the tools!

Edited to Add: After completely starting a new Unity project (and deleting the previous one and salting the earth where it trod), everything is working fine again.

Back to my original question, though: What gives? Was this Unity or VS2012? Or me? I tried so many things that I don't know if something helped and what it was.

Intellisense (C++) is stored in sdf files. Delete them. Also check if you have a custom path in your options on where Intellisense database is stored. If so, check that this path exists and that it has enough space (and write access).

Also, disable external plugins. They could be causing a conflict.

Thanks, Matias -- I take a peek for .SDF files if this crops up again. I'm happy to say that VS2012 is completely cooperating with me for the moment.

You shouldn't have to delete the databases anymore. See http://www.gamedev.net/topic/658549-visual-studio-2013-express-desktop-intellisense-stopped-working/

Just rescan the solution.

That didn't work, either -- tried it early yesterday. As I said above, I had to eliminate the offending project and start fresh before it worked.