SKETCH_PLANE_PARAM read only

Hi,

is there a way to set the work plane for an element via the API. Following code doesn't work:

Parameter p = c.get_Parameter(BuiltInParameter.SKETCH_PLANE_PARAM);
if (p != null)
{
    p.Set(r.Name);
} 

as this parameter is read only. Who knows why?

Any ideas?

Thanks

Christian