Aah, ok! So I create two files Gtk2CellRenderer.cs and Gtk3CellRenderer.cs and CanvasRenderer derives from GtkCellRenderer. Ok!

How do you think about creating a Subfolder in both projects (like Xwt.GtkBackend.GtkWidgets) and move all real Gtk Widgets there? So the Xwt.Backend sources will contain only the IBackend implementations and all real Gtk widgets would be concentrated in one place (no matter if they have different implementation for Gtk2/3 or not).