IReadOnlyList<IElement> GetSelectedElements()

//Report the event args when a layout's selection has changed.

//At 2.x - ArcGIS.Desktop.Layouts.Events.LayoutSelectionChangedEvent.Subscribe((args) => {
//  var elements = args.Elements;
//  string elemNames = "null";
//  if (elements != null && elements.Count() > 0)
//  {
//    elemNames = string.Join(",", elements.Select(e => e.Name).ToList());
//  }
//  System.Windows.MessageBox.Show("LayoutSelectionChangedEvent:" +
//  Environment.NewLine +
//  "   arg Elements: " + elemNames.ToString() +
//  Environment.NewLine +
//  "   arg Layout: " + args.Layout.Name);
//});

ArcGIS.Desktop.Layouts.Events.ElementEvent.Subscribe((args) =>
{
  if (args.Hint == ElementEventHint.SelectionChanged)
  {
    if (args.Container.GetSelectedElements()?.Count() > 0)
    {
      var elemNames = string.Join(",", args.Container.GetSelectedElements().Select(e => e.Name).ToList());
      System.Diagnostics.Debug.WriteLine($"ElementEventHint.SelectionChanged: {elemNames}");
    }
  }

});

//Report the event args when an element has been updated.

//At 2.x - ArcGIS.Desktop.Layouts.Events.ElementsUpdatedEvent.Subscribe((args) => 
//{
//  var elements = args.Elements;
//  string elemNames = "null";
//  if (elements != null && elements.Count() > 0)
//  {
//    elemNames = string.Join(",", elements.Select(e => e.Name).ToList());
//  }
//  System.Windows.MessageBox.Show("ElementsUpdatedEvent:" +
//  Environment.NewLine +
//  "   arg Elements: " + elemNames.ToString() +
//  Environment.NewLine +
//  "   arg Layout: " + args.Layout.Name);
//});
ArcGIS.Desktop.Layouts.Events.ElementEvent.Subscribe((args) =>
{
  string elemNames = "null";
  //can be null if the name was changed
  if (args.Elements?.Count() > 0)
  {
    elemNames = string.Join(",", args.Elements.Select(e => e.Name).ToList());
  }
  else if (args.Container.GetSelectedElements()?.Count() > 0)
  {
    elemNames = string.Join(",", args.Container.GetSelectedElements().Select(e => e.Name).ToList());
  }

  if (args.Hint == ElementEventHint.PropertyChanged)
  {
    System.Diagnostics.Debug.WriteLine($"ElementEventHint.PropertyChanged: {elemNames}");
  }
});