I have a working draft for msbuild for #openbsd . It's a beast of a port, and would benefit from more eyes on it so I'm sharing the draft:

https://github.com/jasperla/openbsd-wip/tree/master/devel/msbuild

MSBuild is the build system used for modern .NET/C# projects. Mono's own xbuild can't handle more recent ones.

I built github.com/SteamRE/Depotdownloader with it successfully; had to use `nuget restore` first though.

Other projects of interest:
- nuget (may require dotnet-cli)
- recent versions of fsharp
- monodevelop
- etc.