sudo apt install apt-transport-https dirmngr gnupg ca-certificates
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb
https://download.mono-project.com/repo/debian stable-raspbianbuster main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
sudo apt update
sudo apt-get update && sudo apt-get upgrade
sudo apt-get install -y mono-complete
sudo apt autoremove
At the time of writing (18 Sep 2019), this gets Mono v6.0.0.334, which works with Buster (RaspBIAN 10). However, there have been reports of incompatabilities which require use of an older version of Mono. These may have now been fixed, or alternatively may be related to use of locales other than en_GB.UTF-8 . Please see other threads in Support Forum for discussions. NOTE: 29 Feb 2020: added -y into the line sudo apt-get install -y mono-complete . This makes the install bypass the usual 'Continue Y/n?' prompt¨which was causing strange problems for some, e.g. worked if just pressed 'Enter' to accept default 'Y', but aborted installation if pressed 'Y Enter'. Bizarre. Reboot your Raspberry Pi This would be a reasonable time to reboot your Pi:
nope they added -y but still no work.