public class ExportSceneContentsFormat 

public void ExportSceneContents()
{
  // Validate the current active view. Only a local scene can be exported.
  bool CanExportScene3DObjects = MapView.Active?.ViewingMode == MapViewingMode.SceneLocal;
  if (CanExportScene3DObjects)
  {
    //Gets the active map view.
    MapView mapView = MapView.Active;
    // Create a scene content export format, export the scene context as a stereolithography format
    var exportFormat = new STLExportSceneContentsFormat()
    {
      Extent = mapView.Extent, // sets Extent property
      FolderPath = @"C:\Temp", // sets FolderPath property
      FileName = "my-3d-objects.stl", //sets FileName property
      IsSingleFileOutput = true // sets whether to export to one single file
    };
    // Export the scene content as 3D objects
    mapView.ExportScene3DObjects(exportFormat);
  }
}