public IList<Item> Items {get;}

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

bool? ok = addToMapDialog.ShowDialog();

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

// cref: OpenItemDialog;ArcGIS.Desktop.Catalog.ItemDialog.Filter
// cref: OpenItemDialog;ArcGIS.Desktop.Catalog.ItemDialog.InitialLocation
// cref: OpenItemDialog;ArcGIS.Desktop.Catalog.ItemDialog.Title
// cref: OpenItemDialog;ArcGIS.Desktop.Catalog.OpenItemDialog
#region OpenItemDialog

/// Adds a single item to a map
OpenItemDialog addToMapDialog = new OpenItemDialog()
{
  Title = "Add To Map",
  InitialLocation = @"C:\Data\NewYork\Counties\Erie\Streets",
  Filter = ItemFilters.Composite_AddToMap
};