public class ElementsPlacementChangedEvent : ArcGIS.Core.Events.CompositePresentationEvent<ElementsUpdatedEventArgs> 

//Report the event args when an element's placement properties are changed.

ArcGIS.Desktop.Layouts.Events.ElementsPlacementChangedEvent.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("ElementsPlacementChangedEvent:" +
  Environment.NewLine +
  "   arg Elements: " + elemNames.ToString() +
  Environment.NewLine +
  "   arg Layout: " + args.Layout.Name);
});