public bool MultiSelect {get; set;}

{
  OpenItemDialog addToProjectDialog = new()
  {
    Title = "Add To Project",
    InitialLocation = @"C:\Data\NewYork\Counties\Maps",
    MultiSelect = true,
    Filter = ItemFilters.Composite_Maps_Import
  };

  bool? ok = addToProjectDialog.ShowDialog();

  if (ok == true)
  {
    IEnumerable<Item> selectedItems = addToProjectDialog.Items;
    foreach (Item selectedItem in selectedItems)
      MapFactory.Instance.CreateMapFromItem(selectedItem);
  }
}