public void FlashFeature( 
   IReadOnlyDictionary<BasicFeatureLayer,List<long>> features
)

public Task FlashSelectedFeaturesAsync()
{
  return QueuedTask.Run(() =>
  {
    //Get the active map view.
    var mapView = MapView.Active;
    if (mapView == null)
      return;

    //Get the selected features from the map and filter out the standalone table selection.
    var selectedFeatures = mapView.Map.GetSelection()
      .Where(kvp => kvp.Key is BasicFeatureLayer)
      .ToDictionary(kvp => (BasicFeatureLayer)kvp.Key, kvp => kvp.Value);

    //Flash the collection of features.
    mapView.FlashFeature(selectedFeatures);
  });

}