public string URI {get;}

{
    //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);
}