public void FlashFeature( 
   SelectionSet 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.

    //At 2.x
    //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);

    var selectedFeatures = mapView.Map.GetSelection();

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

internal class CustomIdentify : MapTool
{
  public CustomIdentify()
  {
    IsSketchTool = true;
    SketchType = SketchGeometryType.Rectangle;

    //To perform a interactive selection or identify in 3D or 2D, sketch must be created in screen coordinates.
    SketchOutputMode = SketchOutputMode.Screen;
  }

  protected override Task<bool> OnSketchCompleteAsync(Geometry geometry)
  {
    return QueuedTask.Run(() =>
    {
      var mapView = MapView.Active;
      if (mapView == null)
        return true;

      //Get all the features that intersect the sketch geometry and flash them in the view. 
      var results = mapView.GetFeatures(geometry);
      mapView.FlashFeature(results);

      var debug = String.Join("\n", results.ToDictionary()
                    .Select(kvp => String.Format("{0}: {1}", kvp.Key.Name, kvp.Value.Count())));
      System.Diagnostics.Debug.WriteLine(debug);
      return true;
    });
  }
}