super(cast(GtkApplicationWindow*)gtk_builder_get_object(builder.getBuilderStruct(),"mainwindow"));

where does gtk_builder_get_object comes from? I see that it's the original c function but I don't see where that binding is defined in gtkD.

I'm doing this with works good enough.

auto s = builder.getObject("mainWindow");
assert(s !is null);
this(cast(GtkApplicationWindow*)s.getObjectGStruct());