I haven't found any concise explanation of this.
So you're looking for a package containing a file called
System.Windows.Forms.dll. You can search:
- on your machine:
apt-file search System.Windows.Forms.dll(the
apt-filepackage must be installed)
- online: at packages.ubuntu.com.
Both methods lead you to (as of Ubuntu 14.04):
Install it with:
sudo apt-get install libmono-system-windows-forms4.0-cil
Found it via this link:
% apt-get install libmono-winforms1.0-cil libmono-winforms2.0-cil