$ cat /etc/apt/sources.list.d/mono-official-stable.list
deb https://download.mono-project.com/repo/ubuntu stable-bionic main
$ cat /etc/issue
Ubuntu 18.04 LTS \n \l
$ sudo apt install mono-devel
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
mono-devel : Depends: libglib2.0-0 (>= 2.56.3) but 2.56.1-2ubuntu1 is to be installed
Recommends: mono-csharp-shell but it is not going to be installed
Recommends: referenceassemblies-pcl but it is not going to be installed
Recommends: msbuild but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
$ dpkg --get-selections | grep hold
no output
