_topologyExt.MapTopology.AddClass(_featClass)
TypeOf _featClass Is ITopologyClass
UID extUid = new UIDClass(); extUid.Value = "esriEditorExt.TopologyExtension"; ITopologyExtension topologyExt = ArcMap.Application.FindExtensionByCLSID(extUid) as ITopologyExtension; IMapTopology2 mt2 = topologyExt.MapTopology as IMapTopology2; ArcMap.Application.StatusBar.set_Message(0, mt2.LayerCount.ToString());