编辑时snapping的添加

注意需要在编辑模式下进行snapping的添加（也即先需要使用IEngineEditor进入编辑状态）：
IMapControl3 mMap = (IMapControl3)((IToolbarControl)toolHook).Buddy;
IEngineEditor m_engineEditor = new EngineEditorClass();
if (m_engineEditor.EditState == esriEngineEditState.esriEngineStateNotEditing)
{
    //Start editing the workspace of the first feature layer found.        
    ILayer currentLayer = mMap.get_Layer(0);
    if (currentLayer is IFeatureLayer)
    {
        IFeatureLayer featureLayer = currentLayer as IFeatureLayer;
        IDataset dataset = featureLayer.FeatureClass as IDataset;
        IWorkspace workspace = dataset.Workspace;
        m_engineEditor.StartEditing(workspace, mMap.Map);
        ((IEngineEditLayers)m_engineEditor).SetTargetLayer(featureLayer, 0);
    }
    //snapping
    if (mMap.LayerCount != 0)
    {
        IEngineEditor editor = new EngineEditorClass();
        IEngineEditLayers editLayers = editor as IEngineEditLayers;
        IEngineSnapEnvironment snapEnvironment = editor as IEngineSnapEnvironment;

        IEngineFeatureSnapAgent featureSnapAgent = new EngineFeatureSnap();
        featureSnapAgent.FeatureClass = ((IFeatureLayer)mMap.get_Layer(0)).FeatureClass; ;
        featureSnapAgent.HitType = esriGeometryHitPartType.esriGeometryPartVertex | esriGeometryHitPartType.esriGeometryPartBoundary | esriGeometryHitPartType.esriGeometryPartEndpoint;
        snapEnvironment.AddSnapAgent(featureSnapAgent);                    
    }

}