I installed the GtkSharp 3.22.24.36 package via Project > Add NuGet Packages..., which completed successfully. But now Monodevelop can't decide which version of GtkSharp to use (3.22.24.36 or the in-built one, 2.12).
If I uncheck gtk-sharp
in the Edit references... dialog, it tells me that I can't use the UI designer without it.
How do I get Monodevelop to use the downloaded GtkSharp (3.22)?
MonoDevelop comes with a custom version of Gtk2 + Xwt bundled. If you plan to use Gtk3, then the designer must be deactivated, since it won't work with Gtk3.
My advice, anyway, no matter which toolkit you use, is to avoid using the designer. As soon as you involve the designer, you code heavily depends on the IDE you use (Visual Studio, MonoDevelop, NetBeans... you name it).
Creating user interfaces "by hand" is no longer traumatic as it was in the 90's with the Windows API. For example, you can find a very good Gtk# tutorial in ZetCode.
User Interface Toolkits are actually very similar, they change the name of widgets and sometimes provide a slightly different layout, but they are all mostly the same, no matter it is WinForms or Gtk(for C#), Swing (Java), or Qt (C++ and others).