In my experience, SnappingManager does not seem to work with ArcGISDynamicMapServiceLayer (Maybe there's another way to do this?) What you could do (what I did) is loop through your ArcGISDynamicMapServiceLayer/DLayer's layerInfos and retrieve all the layer IDs and Names (DLayer.layerInfos.name, DLayer.layerInfos.id) then for each one, you can create a new FeatureLayer() object. eg. new FeatureLayer(featureLayerUrl, { id: featureLayerId, outFields: ["*"] }) for the featureLayerUrl you can use the same url used for your ArcGISDynamicMapServiceLayer but append ( "/" + DLayer.layerInfos.id ) for the featureLayerId use DLayer.layerInfos.name then with each of these FeatureLayers, you can add them to your map object and to your snappmanager's layerInfos.
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