ubuntu mono env issues resolved (sort of)

Yesterday I found that I could no longer build and run my .net project on ubuntu, after upgrading to 16.04. On investigation this is because my mono environment was also upgraded (to a 4.xxx build). In 4.xxx versions, mono has discontinued support for .NET 3.5. That's awkward, since I'm stuck on 3.5 probably forever (a script upgrade to 4.6 still hasn't even gotten on the Unity technology roadmap).

I tried for most of the day to downgrade to mono 3.12.0. In principle this should be possible. The mono website contains a line about how to do this, which I tried and failed--I added the correct snapshot as my tracked xamarin repository, but I must have mono available in some ubuntu repo as well, because a simple install of mono-complete still added the latest version.

I also found a blog post purporting to explain how to do it. It involved pre-installing a certain set of dependencies, and then installing mono-complete with '-t' to indicate the old version. This also failed in at least two ways.

I wandered down some dark paths trying to find ways to install all the dependencies for old-mono on my machine. Eventually, I more or less gave up, removed all the mono packages, and then installed them back at the newest revision. When I idly tried rebuilding my .net project, it worked!

What seems to be happening is, even though mono 4.xxx no longer supports .net 3.5, it still ships with API stubs for backwards compatibility. Somehow I didn't get them when I did my OS upgrade from 15.10->16.04. But having reinstalled mono (several times, heh), these stubs showed up, letting me build against .NET 3.5, even though monodevelop lists this platform as "uninstalled". Running the unittests works, but it must be going against the 4.5 runtime (as that's all it has).

So I'm back in business, but I don't exactly have warm fuzzies about this. I'd really like to have cross-platform support for developing in this project, but if problems like this keep turning up, the cost may be too high.