public class LasPointSelectionFilter 

{
  // must be on MCT 

  // create the filter
  var filter = new LasPointSelectionFilter();
  // set the filter geometry
  // don't set VisiblePoints, ClassCodes - use the existing layer values
  filter.FilterGeometry = polygon;

  // perform the selection
  var selCount = await lasDatasetLayer.SelectAsync(filter, SelectionCombinationMethod.New);

  // set up a second filter and configure the VisiblePoints, ClassCodes
  // note that the ClassCodes is using only (4,5)
  // whereas the layers setting is (3,4,5)
  var filter2 = new LasPointSelectionFilter();
  filter2.VisiblePoints = true;
  filter2.ClassCodes = new List<int>() { 4, 5 };
  filter2.FilterGeometry = polygon;

  // perform the selection
  selCount = await lasDatasetLayer.SelectAsync(filter2, SelectionCombinationMethod.New);
}