mono-complete and mono-develop can't compile within ide

Install the two packages:
$ apt install mono-complete
$ apt install monodevelop

on a fresh Ubuntu 16.04 box. Load an existing project that compiles and works with monodevelop and mono-complete as provided by Xamarin via their website.

Installed is:
monodevelop/xenial,xenial,now 5.10.0.871-2 all [installed]
  Development Environment for GNOME
mono-complete/xenial,now 4.2.1.102+dfsg2-5ubuntu1 amd64 [installed]
  complete Mono runtime, development tools and all libraries

Compiling with in the monodevelop ide results in CSC errors.
* Got to Build -> Build All

A number of errors occur such as:
/home/jason/Programming/serialportstream/code/CSC: Error CS0518: The predefined type `System.Object' is not defined or imported (CS0518) (SerialPortStream)

This is a standard type, so it appears there are some build/packaging issues with the version that comes from ubuntu sources.

Solution:
* For now, you have to take those from Mono itself:
# apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
# echo "deb http://download.mono-project.com/repo/debian wheezy main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
# apt-get update

Then everything works as expected.

root@ubuntu1604x64:/etc/default# lsb_release -rd
Description: Ubuntu Xenial Xerus (development branch)
Release: 16.04

root@ubuntu1604x64:/etc/default# apt-cache policy monodevelop
monodevelop:
  Installed: 5.10.0.871-2
  Candidate: 5.10.0.871-2
  Version table:
 *** 5.10.0.871-2 500
        500 http://de.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        500 http://de.archive.ubuntu.com/ubuntu xenial/universe i386 Packages
        100 /var/lib/dpkg/status
root@ubuntu1604x64:/etc/default# apt-cache policy mono-complete
mono-complete:
  Installed: 4.2.1.102+dfsg2-5ubuntu1
  Candidate: 4.2.1.102+dfsg2-5ubuntu1
  Version table:
 *** 4.2.1.102+dfsg2-5ubuntu1 500
        500 http://de.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: mono-complete 4.2.1.102+dfsg2-5ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-15.31-generic 4.4.6
Uname: Linux 4.4.0-15-generic x86_64
ApportVersion: 2.20.1-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Apr 2 20:22:53 2016
InstallationDate: Installed on 2016-03-28 (5 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Beta amd64 (20160323)
SourcePackage: mono
UpgradeStatus: No upgrade log present (probably fresh install)