[Gtk-sharp-list] Cairo context disposing pattern

[pinggi at centrum.cz]

Hi, 

I encountered one strangeness:

			Cairo.Context ctx = new Cairo.Context(cairoImageSurface);
			((IDisposable)ctx).Dispose();

Why I need to cast cairo context to IDisposable to be able to call Dispose method?
What is the reason, because I would expect something like this:

			Cairo.Context ctx = new Cairo.Context(cairoImageSurface);
			ctx.Dispose();

[pinggi]