public sealed class TinDatasetDefinition : ArcGIS.Core.Data.Definition, System.IDisposable  

public async Task GetTinDatasetDefinition()
{
  try
  {
    await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
    {
      string path = @"d:\Data\Tin";
      var fileConnection = new FileSystemConnectionPath(new Uri(path), FileSystemDatastoreType.Tin);

      using (FileSystemDatastore dataStore = new FileSystemDatastore(fileConnection))
      {
        // TIN is in a folder at d:\Data\Tin\TinDataset

        string dsName = "TinDataset";

        using (var def = dataStore.GetDefinition<ArcGIS.Core.Data.Analyst3D.TinDatasetDefinition>(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();
  }
}