public class SurfaceLayer : Layer, ArcGIS.Desktop.Core.IMetadataInfo, IMetadataSource, System.ComponentModel.INotifyPropertyChanged  

// find the first TIN layer
var tinLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<TinLayer>().FirstOrDefault();

// find the first Terrain layer
var terrainLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<TerrainLayer>().FirstOrDefault();

// find the first LAS dataset layer
var lasDatasetLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<LasDatasetLayer>().FirstOrDefault();

// find the set of surface layers
var surfacelayers = MapView.Active.Map.GetLayersAsFlattenedList().OfType<SurfaceLayer>();