public Size GetViewSize()

{
  var sname = featSceneLayer.Name;

  // Note: call within QueuedTask.Run()
  {
    //Select all features within the current map view
    var sz = MapView.Active.GetViewSize();

    var c_ll = new Coordinate2D(0, 0);
    var c_ur = new Coordinate2D(sz.Width, sz.Height);
    //Use screen coordinates for 3D selection on MapView
    var env = EnvelopeBuilderEx.CreateEnvelope(c_ll, c_ur);

    //HasAssociatedFeatureService does not matter for SelectFeaturesEx
    //or GetFeaturesEx
    var result = MapView.Active.SelectFeaturesEx(env);
    //var result = MapView.Active.GetFeaturesEx(env);

    //The list of object ids from SelectFeaturesEx
    var oids1 = result.ToDictionary().Where(kvp => kvp.Key.Name == sname).First().Value;
    //TODO - use the object ids

    MapView.Active.Map.ClearSelection();
  }
}

// Note: call within QueuedTask.Run()
{
  if (!featSceneLayer.HasAssociatedFeatureService)
    return;//no search or select

  //Select all features within the current map view
  var sz = MapView.Active.GetViewSize();
  var map_pt1 = MapView.Active.ClientToMap(new System.Windows.Point(0, sz.Height));
  var map_pt2 = MapView.Active.ClientToMap(new System.Windows.Point(sz.Width, 0));

  //Convert to an envelope
  var temp_env = EnvelopeBuilderEx.CreateEnvelope(map_pt1, map_pt2, MapView.Active.Map.SpatialReference);

  //Project if needed to the layer spatial ref
  SpatialReference sr = null;
  using (var fc = featSceneLayer.GetFeatureClass())
  using (var fdef = fc.GetDefinition())
    sr = fdef.GetSpatialReference();

  var env = GeometryEngine.Instance.Project(temp_env, sr) as Envelope;

  //Set up a query filter
  var sf = new SpatialQueryFilter()
  {
    FilterGeometry = env,
    SpatialRelationship = SpatialRelationship.Intersects,
    SubFields = "*"
  };

  //Select against the feature service
  var select = featSceneLayer.Select(sf);
  if (select.GetCount() > 0)
  {
    //enumerate over the selected features
    using (var rc = select.Search())
    {
      while (rc.MoveNext())
      {
        using (var feature = rc.Current as Feature)
        {
          oid = feature.GetObjectID();
          //etc.
        }
      }
    }
  }
  MapView.Active.Map.ClearSelection();
}