“The Type or Namespace ‘GTK’ could not be found” error – MonoDevelop on Linux
When you press F5 to build and get this error in MonoDevelop, just go to:
Project -> Your Project name Options -> Build -> General
You probably see:
Target Framework: Mono /.Net 3.5
Change it to:
Target Framework: Mono /.Net 4.0
Do all what MonoDevelop wants from you. Now, your project has no errors during building.