I've been working on a .NET 3.5 C# project using Visual Studio Community 2015, but I had always intended to do most of the development on Linux (Ubuntu Gnome 15.04) using Mono and MonoDevelop.
I'm running the latest stable Mono release (4.0.4) and MonoDevelop (5.9.6), which supports .NET 4.5 and can open the VS created solution file without issue.
Now I've been writing C++ apps on Linux for the best part of a decade, but C# and .NET are completely new to me. So I assumed that if I installed a Mono version that supports .NET 4.5, I would get .NET 3.5 as well because the later version is a superset of the older - much like a C++14 compiler supports C++03.
However this doesn't seem to be the case, as MonoDevelop states (under the Target Framework option for each project):
No configuration issue, Mono dropped support for the older frameworks in the 4.x release. If you need to compile against the 3.5 assemblies, then yes, you will need a parallel install.
We no longer build the reference assemblies for the .NET 2.0, .NET 3.5 or .NET 4.0 APIs, we now ship binaries of the reference assemblies (API contracts, without any actual executable code in them).
Mono will now only build the .NET 4.5 assemblies as well as the mobile-based profiles.
Note: You can still run assemblies compiled for earlier .NET profiles on Mono, there’s no need to recompile them (they’ll just run on the .NET 4.5 assemblies instead).