public long GetCount()

{
  //Must be called within QueuedTask.Run()
  void GetCount(FeatureLayer featureLayer)
  {
    // Row count in a Table
    Table table = featureLayer.GetTable();
    long count = table.GetCount();

    // Feature count in a FeatureClass
    FeatureLayer lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
    FeatureClass featureClass = lyr.GetFeatureClass();
    count = featureClass.GetCount();
  }
}