public int SelectionCount {get;}

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

await 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;

});