public List<int> ClassCodes {get; set;}

{
  // search within an extent
  ArcGIS.Core.Data.Analyst3D.LasPointFilter pointFilter = new ArcGIS.Core.Data.Analyst3D.LasPointFilter();
  pointFilter.FilterGeometry = envelope;
  using (ArcGIS.Core.Data.Analyst3D.LasPointCursor ptCursor = lasDataset.SearchPoints(pointFilter))
  {
    while (ptCursor.MoveNext())
    {
      using (ArcGIS.Core.Data.Analyst3D.LasPoint point = ptCursor.Current)
      {

      }
    }
  }

  // search within an extent and limited to specific classification codes
  pointFilter = new ArcGIS.Core.Data.Analyst3D.LasPointFilter();
  pointFilter.FilterGeometry = envelope;
  pointFilter.ClassCodes = new List<int> { 4, 5 };
  using (ArcGIS.Core.Data.Analyst3D.LasPointCursor ptCursor = lasDataset.SearchPoints(pointFilter))
  {
    while (ptCursor.MoveNext())
    {
      using (ArcGIS.Core.Data.Analyst3D.LasPoint point = ptCursor.Current)
      {

      }
    }
  }
}

{
  // Note: Needs QueuedTask to run
  {
    // searching on the LasDatasetLayer will honor any LasPointDisplayFilter
    // search all points
    using (ArcGIS.Core.Data.Analyst3D.LasPointCursor ptCursor = lasDatasetLayer.SearchPoints(null))
    {
      while (ptCursor.MoveNext())
      {
        using ArcGIS.Core.Data.Analyst3D.LasPoint point = ptCursor.Current;
        //Use the point
      }
    }

    // search within an extent
    ArcGIS.Core.Data.Analyst3D.LasPointFilter pointFilter = new ArcGIS.Core.Data.Analyst3D.LasPointFilter();
    pointFilter.FilterGeometry = envelope;
    using (ArcGIS.Core.Data.Analyst3D.LasPointCursor ptCursor = lasDatasetLayer.SearchPoints(pointFilter))
    {
      while (ptCursor.MoveNext())
      {
        using ArcGIS.Core.Data.Analyst3D.LasPoint point = ptCursor.Current;
        //Use the point
      }
    }

    // search within an extent and limited to specific classification codes
    pointFilter = new ArcGIS.Core.Data.Analyst3D.LasPointFilter();
    pointFilter.FilterGeometry = envelope;
    pointFilter.ClassCodes = new List<int> { 4, 5 };
    using (ArcGIS.Core.Data.Analyst3D.LasPointCursor ptCursor = lasDatasetLayer.SearchPoints(pointFilter))
    {
      while (ptCursor.MoveNext())
      {
        using ArcGIS.Core.Data.Analyst3D.LasPoint point = ptCursor.Current;
        //Use the point
      }
    }
  }
}