public int SelectionCount {get;}

{
  if (featSceneLayer.HasAssociatedFeatureService)
    return;//Search and Select not supported

  // Note: call within QueuedTask.Run()
  {
    QueryFilter qf = new QueryFilter()
    {
      ObjectIDs = new List<long>() { 6069, 6070, 6071 },
      SubFields = "*"
    };
    featSceneLayer.Select(qf, SelectionCombinationMethod.New);

    featSceneLayer.HideSelectedFeatures();
    var selectionCount = featSceneLayer.SelectionCount;

    featSceneLayer.ShowHiddenFeatures();
    selectionCount = featSceneLayer.SelectionCount;
  }
}