public MapFrame ActivatedMapFrame {get;}

//The active view must be a layout view.
var lv = LayoutView.Active;
if (lv == null)
  return;
var layout = lv.Layout;
if (layout == null)
  return;

//Deactivate any activated map frame
//Note: we are on the UI thread!
lv.DeactivateMapFrame();//no-op if nothing activated

//or - check if a  map frame is activated first...
if (lv.ActivatedMapFrame != null)
  //Note: we are on the UI thread!
  lv.DeactivateMapFrame();

//The active view must be a layout view.
var lv = LayoutView.Active;
if (lv == null)
  return;

var map_view = lv.ActivatedMapView;
if (map_view != null)
{
  //TODO - use activated map view
}
var map_frame = lv.ActivatedMapFrame;
if (map_frame != null)
{
  //TODO - use activated map frame
}