public sealed class Terrain : ArcGIS.Core.Data.Dataset, System.IDisposable  

public async Task OpenTerrain()
{
  try
  {
    await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
    {
      string path = @"d:\Data\Terrain\filegdb_Containing_A_Terrain.gdb";
      var fileConnection = new FileGeodatabaseConnectionPath(new Uri(path));

      using (Geodatabase dataStore = new Geodatabase(fileConnection))
      {
        string dsName = "nameOfTerrain";

        using (var dataset = dataStore.OpenDataset<ArcGIS.Core.Data.Analyst3D.Terrain>(dsName))
        {
        }
      }
    });
  }
  catch (GeodatabaseNotFoundOrOpenedException exception)
  {
    // Handle Exception.
  }
}

//Must be on the QueuedTask.Run()

Envelope extent;
SpatialReference sr;
using (var tin = tinLayer.GetTinDataset())
{
  using (var tinDef = tin.GetDefinition())
  {
    extent = tinDef.GetExtent();
    sr = tinDef.GetSpatialReference();
  }
}

using (var terrain = terrainLayer.GetTerrain())
{
  using (var terrainDef = terrain.GetDefinition())
  {
    extent = terrainDef.GetExtent();
    sr = terrainDef.GetSpatialReference();
  }
}

using (var lasDataset = lasDatasetLayer.GetLasDataset())
{
  using (var lasDatasetDef = lasDataset.GetDefinition())
  {
    extent = lasDatasetDef.GetExtent();
    sr = lasDatasetDef.GetSpatialReference();
  }
}

//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);