public override Nullable<bool> ShowDialog()

{
  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);
  }
}

{
  var openDlg = new OpenItemDialog
  {
    Title = "Select a Feature Class",
    InitialLocation = @"C:\Data",
    MultiSelect = false,
    BrowseFilter = BrowseProjectFilter.GetFilter(ItemFilters.GeodatabaseItems_All)
  };
  //show the browse dialog
  bool? ok = openDlg.ShowDialog();
  if (!ok.HasValue || openDlg.Items.Count == 0)
  {
    // nothing selected, leave
  }

  // Note: Needs QueuedTask to run
  // get the item
  var item = openDlg.Items.First();
  // see if the item has a dataset
  if (ItemFactory.Instance.CanGetDataset(item))
  {
    // get it
    using var ds = ItemFactory.Instance.GetDataset(item);
    // access some properties
    var name = ds.GetName();
    var path = ds.GetPath();
    // if it's a featureclass
    if (ds is FeatureClass fc)
    {
      // create a layer 
      var featureLayerParams = new FeatureLayerCreationParams(fc)
      {
        MapMemberIndex = 0
      };
      var layer = LayerFactory.Instance.CreateLayer<FeatureLayer>(featureLayerParams, MapView.Active.Map);

      // continue
    }
  }
}

{
  BrowseProjectFilter portalAndLocalProjectsFilter = new();
  //A filter to pick projects from the portal
  //This filter will allow selection of ppkx and portal project items on the portal
  portalAndLocalProjectsFilter.AddFilter(BrowseProjectFilter.GetFilter("esri_browseDialogFilters_projects_online_proprojects"));
  //A filter to pick projects from the local machine
  portalAndLocalProjectsFilter.AddFilter(BrowseProjectFilter.GetFilter("esri_browseDialogFilters_projects"));
  //Create the OpenItemDialog and set the filter to the one we just created
  var openDlg = new OpenItemDialog()
  {
    Title = "Select a Project",
    MultiSelect = false,
    BrowseFilter = portalAndLocalProjectsFilter
  };
  //Show the dialog
  var result = openDlg.ShowDialog();
  //Check if the user clicked OK and selected an item
  bool? ok = openDlg.ShowDialog();
  if (!ok.HasValue || openDlg.Items.Count == 0)
    return; //nothing selected
  var selectedItem = openDlg.Items.FirstOrDefault();
  //Open the project use the OpenAsync method.
}