public List<LasReturnType> Returns {get; set;}

// get the current display filter
LasPointDisplayFilter ptFilter = lasDatasetLayer.GetDisplayFilter();


// display only ground points
lasDatasetLayer.SetDisplayFilter(LasPointDisplayFilterType.Ground);

// display first return points
lasDatasetLayer.SetDisplayFilter(LasPointDisplayFilterType.FirstReturnPoints);

// set display filter to a set of classification codes
List<int> classifications = new List<int>() { 4, 5, 7, 10 };
lasDatasetLayer.SetDisplayFilter(classifications);

// set display filter to a set of returns
List<ArcGIS.Core.Data.Analyst3D.LasReturnType> returns = new List<ArcGIS.Core.Data.Analyst3D.LasReturnType>()
        { ArcGIS.Core.Data.Analyst3D.LasReturnType.ReturnFirstOfMany};
lasDatasetLayer.SetDisplayFilter(returns);

// set up a display filter
var newDisplayFilter = new LasPointDisplayFilter();
newDisplayFilter.Returns = new List<ArcGIS.Core.Data.Analyst3D.LasReturnType>()
        { ArcGIS.Core.Data.Analyst3D.LasReturnType.ReturnFirstOfMany, ArcGIS.Core.Data.Analyst3D.LasReturnType.ReturnLastOfMany};
newDisplayFilter.ClassCodes = new List<int>() { 2, 4 };
newDisplayFilter.KeyPoints = true;
newDisplayFilter.WithheldPoints = false;
newDisplayFilter.SyntheticPoints = false;
newDisplayFilter.NotFlagged = false;
lasDatasetLayer.SetDisplayFilter(returns);