You need Mono version 5.10.0.160

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
sudo apt install apt-transport-https
echo "deb https://download.mono-project.com/repo/ubuntu xenial/snapshots/5.10.0.160 main" | sudo tee /etc/apt/sources.list.d/mono-official-preview.list

sudo apt-get update
sudo apt-get install mono-devel

How to install mono-devel on other Linux distributions.

sudo apt-get purge libmono* cli-common mono-runtime
sudo apt-get autoremove

WARNING: DO NOT UPDATE MONO FOR NOW.USE ONLY Version 5.10.0.125 or 5.10.0.160.