IMapTopology里面总是空的是什么问题？

为什么这段代码获取的IMapTopology里面总是没有层，ClassCount为0。明明MapTopology已经加层了。
UID mUID = new UIDClass();
mUID.Value = "esriEditorExt.TopologyExtension";
ITopologyExtension topoExt = m_application.FindExtensionByCLSID(mUID) as ITopologyExtension;
IMapTopology mapTopo = topoExt.MapTopology as IMapTopology;
IFeatureClass featClass = oBJClass as IFeatureClass;
int lID = mapTopo.FindClass(featClass);
lID是-1