string id = "GeofenceId";

var position = new BasicGeoposition();
position.Latitude = 40;
position.Longitude = -120;
var geocircle = new Geocircle(position, 100);

var mask = MonitoredGeofenceStates.Entered | MonitoredGeofenceStates.Exited;

var area = new Geofence(id, geocircle, mask, false);

GeofenceMonitor.Current.Geofences.Clear();
GeofenceMonitor.Current.Geofences.Add(area);

GeofenceMonitor.Current.GeofenceStateChanged += (GeofenceMonitor obj, object args) =>
{
  var reports = obj.ReadReports();
  Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
  {
    foreach (GeofenceStateChangeReport report in reports)
    {
      log.AppendLine(report.Geofence.Id);
      log.AppendLine(report.NewState.ToString());
    }
    status.Text = log.ToString();
  });
};