ISnappingEnvironment 接口设置问题

调用接口ISnappingEnvironment实现在草图编辑时捕捉节点，在测试时发现捕捉功能不起作用。下面的代码是ISnappingEnvironment接口设置，除此之外我还需要做什么设置或关联吗
 
ISnappingEnvironment接口设置：
UID guid = new UIDClass();
guid.Value = "{E07B4C52-C894-4558-B8D4-D4050018D1DA}"; //Snapping extension.
IExtension extension = extensionManager.FindExtension(guid);
ISnappingEnvironment m_SnappingEnv = extension as ISnappingEnvironment;
m_SnappingEnv.SnappingType = esriSnappingType.esriSnappingTypeVertex;