public double MinimumLength {get; set;}

{
  // must be on MCT 

  var pipelineFilter = new LasPointPipelineSelectionFilter();

  pipelineFilter.VisiblePoints = true;
  pipelineFilter.ClassCodes = new List<int>();  // empty list means all classification codes
  pipelineFilter.FilterGeometry = polygon;

  // configure a few of the properties and accept defaults for others
  pipelineFilter.ApplyWindCorrection = true;
  pipelineFilter.MinimumLength = 25;  // meters

  var pipelineSelCount = await lasDatasetLayer.SelectAsync(pipelineFilter, SelectionCombinationMethod.New);
}