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

{
  //Note: This snippet requires to be run on the MCT (Main CIM Thread)
  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))
    {
      //Do something with the definition
    }
  }
}

{
  //Note: Needs QueuedTask to run
  {
    using (var tin = tinLayer.GetTinDataset())
    {
      using var tinDef = tin.GetDefinition();
      Envelope extent = tinDef.GetExtent();
      SpatialReference sr = tinDef.GetSpatialReference();
    }

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

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