Install libgtk3.0-cil by entering the following commands in the terminal:
sudo apt update sudo apt install libgtk3.0-cil
Description:
CLI binding for the GTK+ 3.0 toolkit
This package provides the gtk-sharp assembly that allows CLI (.NET) programs to use the GTK+ 3.0 user interface toolkit. . GTK# 3.0 is a CLI (.NET) language binding for the GTK+ 3.0 toolkit and related libraries.
Homepage: http://www.mono-project.com/GtkSharp
Version: 2.99.3-2
Section: universe/cli-mono