Tried this and failed with xbuild step. 124 errors all seem to be related to missing types, eg.

... CSC: error CS0518: The predefined type System.IntPtr' is not defined or imported
...`

When installing with apt-get I get notice that mono-gmcs is missing or obsolete:

Package mono-gmcs is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However, the following packages replace it:
  mono-xbuild mono-mcs mono-devel

mono-devel and mono-mcs both show as installed.

This is on Mint 19 based on Ubuntu 18.04.

I'm not at all familiar with Mono but have good skills in Linux. Is there some way to get this working now?