IEngineFeatureSnapAgent设置捕捉类型有几种不支持吗？？

 IEngineFeatureSnapAgent featureSnapAgent = new EngineFeatureSnap();
                        featureSnapAgent.FeatureClass = (pLayer as IFeatureLayer).FeatureClass;
                        featureSnapAgent.HitType = esriGeometryHitPartType.esriGeometryPartVertex
                              | esriGeometryHitPartType.esriGeometryPartBoundary
                              | esriGeometryHitPartType.esriGeometryPartEndpoint
                              | esriGeometryHitPartType.esriGeometryPartCentroid
                            | esriGeometryHitPartType.esriGeometryPartMidpoint
                            | esriGeometryHitPartType.esriGeometryPartSurface;
                        m_snapEnvironment.AddSnapAgent(featureSnapAgent);
 
 
 
但是                   | esriGeometryHitPartType.esriGeometryPartCentroid
                            | esriGeometryHitPartType.esriGeometryPartMidpoint
                            | esriGeometryHitPartType.esriGeometryPartSurface;
这三种：终点、交点和切线的捕捉效果 没有起作用