public class ActiveLayoutViewChangedEvent : ArcGIS.Core.Events.CompositePresentationEvent<LayoutViewEventArgs> 

//Report the event args when a the layout view has changed.

ArcGIS.Desktop.Layouts.Events.ActiveLayoutViewChangedEvent.Subscribe((args) =>
{
  System.Windows.MessageBox.Show("ActiveLayoutViewChangedEvent:" +
    Environment.NewLine +
    "   arg Layoutview: " + args.LayoutView.Layout.Name +
    Environment.NewLine +
    "   arg Type: " + args.Type.ToString());
});