public string URI {get;}

public static IReadOnlyList<Map> GetMapsFromMapPanes()
{
  //Gets the unique list of Maps from all the MapPanes.
  //Note: The list of maps retrieved from the MapPanes
  //maybe less than the total number of Maps in the project.
  //It depends on what maps the user has actually opened.
  var mapPanes = ProApp.Panes.OfType<IMapPane>()
              .GroupBy((mp) => mp.MapView.Map.URI).Select(grp => grp.FirstOrDefault());
  List<Map> uniqueMaps = new List<Map>();
  foreach (var pane in mapPanes)
    uniqueMaps.Add(pane.MapView.Map);
  return uniqueMaps;
}