public class CIMTerrainDirtyAreaRenderer : CIMTinSimpleRenderer, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

// 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);
}