public class ActivateMapFrameEvent : ArcGIS.Core.Events.CompositePresentationEvent<ActivateMapFrameEventArgs> 

//Report the event args when a map frame is activated or deactivated.

ArcGIS.Desktop.Layouts.Events.ActivateMapFrameEvent.Subscribe((args) =>
{
  System.Windows.MessageBox.Show("ActiveMapFrameEvent:" +
  Environment.NewLine +
  "   arg IsActivated: " + args.IsActivated.ToString() +
  Environment.NewLine +
  "   arg Layout: " + args.Layout.Name +
  Environment.NewLine +
  "   arg MapFrame: " + args.MapFrame.Name);
});