public sealed class TerrainLayer : SurfaceLayer, 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>();

//Must be on the QueuedTask.Run()

string terrainPath = @"d:\Data\Terrain\filegdb_Containing_A_Terrain.gdb";
var terrainURI = new Uri(terrainPath);

var terrainCP = new TerrainLayerCreationParams(terrainURI);
terrainCP.Name = "My Terrain Layer";
terrainCP.IsVisible = false;

//Create the layer to the terrain
var terrainLayer = LayerFactory.Instance.CreateLayer<TerrainLayer>(terrainCP, map);

//Must be on the QueuedTask.Run()

var terrainCP_ds = new TerrainLayerCreationParams(terrain);
terrainCP_ds.Name = "My Terrain Layer";
terrainCP_ds.IsVisible = true;

//Create the layer to the terrain
var terrainLayer_ds = LayerFactory.Instance.CreateLayer<TerrainLayer>(terrainCP_ds, map);

//Must be on the QueuedTask.Run()

var terrainCP_renderers = new TerrainLayerCreationParams(terrain);
terrainCP_renderers.Name = "My LAS Layer";
terrainCP_renderers.IsVisible = true;

// define the edge type renderer - use defaults
var edgeRD = new TinBreaklineRendererDefinition();

// define the face/surface renderer
var faceRD = new TinFaceClassBreaksRendererDefinition();
faceRD.ClassificationMethod = ClassificationMethod.NaturalBreaks;
// accept default color ramp, breakCount

// define the dirty area renderer - use defaults
var dirtyAreaRD = new TerrainDirtyAreaRendererDefinition();

// add renderers to dictionary
var t_dict = new Dictionary<SurfaceRendererTarget, TinRendererDefinition>();
t_dict.Add(SurfaceRendererTarget.Edges, edgeRD);
t_dict.Add(SurfaceRendererTarget.Surface, faceRD);
t_dict.Add(SurfaceRendererTarget.DirtyArea, dirtyAreaRD);

// assign dictionary to creation params
terrainCP_renderers.RendererDefinitions = t_dict;

//Create the layer to the terrain
var terrainLayer_rd = LayerFactory.Instance.CreateLayer<TerrainLayer>(terrainCP_renderers, map);

// applies to Terrain layers only

var dirtyAreaRendererDef = new TerrainDirtyAreaRendererDefinition();
// accept default labels, symbolTemplate

var terrainLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<TerrainLayer>().FirstOrDefault();
if (terrainLayer == null)
  return;

if (terrainLayer.CanCreateRenderer(dirtyAreaRendererDef))
{
  CIMTinRenderer renderer = terrainLayer.CreateRenderer(dirtyAreaRendererDef);
  if (terrainLayer.CanSetRenderer(renderer, SurfaceRendererTarget.DirtyArea))
    terrainLayer.SetRenderer(renderer, SurfaceRendererTarget.DirtyArea);
}