Install libgtk3.0-cil-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libgtk3.0-cil-dev
Description:
CLI binding for the GTK+ toolkit 3.0
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. . This package contains development files for ATK#, Cairo#, GDK#, Glib#, GTK# and Pango#, and should be used for compilation.
Homepage: http://www.mono-project.com/GtkSharp
Version: 2.99.3-2
Section: universe/cli-mono