[Gtk-sharp-list] System.InvalidCastException from generated code

[Mike Kestner]

On Mon, 2004-01-26 at 07:05, Martin Willemoes Hansen wrote:

> > Looking into the code, i found that the generator made the code:
> > 
> > 
> > > 		public Gtk.SourceMarker CreateMarker(string name, string type, Gtk.TextIter where) {
> > > 			IntPtr raw_ret = gtk_source_buffer_create_marker(Handle, name, type, ref where);
> > > 			Gtk.SourceMarker ret = (Gtk.SourceMarker) GLib.Object.GetObject(raw_ret);
> > > 			return ret;
> > > 		}
> 
> try changing the code to be like this:
> Gtk.SourceMarker ret = new Gtk.SourceMarker (raw_ret);

Not a good idea.  If you do this, you will end up with two or more
wrappers pointing to the same native object, but only holding one native
ref to the object.  When one of those wrappers gets GC'd, all your other
wrappers are hosed.  It's going to bite you eventually, and it will be
one of those almost impossible to debug problems, especially for
somebody who doesn't know that you short-circuited the GetObject
mechanism with custom code.

> > I think the problem comes from the folowing C code:
> > 
> > 
> > >  33             GType 
> > >  34             gtk_source_marker_get_type (void)
> > >  35             {
> > >  36             	static GType our_type = 0;
> > >  37             
> > >  38             	/* this function's solely purpose is to allow us to later
> > >  39             	 * derive GtkSourceMarker from other base object and not break
> > >  40             	 * ABI compatibility (since this way the type is resolved at
> > >  41             	 * runtime) */
> > >  42             	if (!our_type) {
> > >  43				our_type = GTK_TYPE_TEXT_MARK;
> > 
> > It seems that Gtk# is confused as to the type of the object. What would
> > be the best way to fix this?

Ben and I talked about this on IRC last night, and I advised him to file
a bug against SourceView for subclassing objects without registering a
new GType for the object.  There is no way a language binding can
properly introspect a GObject type that's coded in this manner.

-- 
Mike Kestner <mkestner@speakeasy.net>