public override Nullable<bool> ShowDialog()

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);
}

BrowseProjectFilter portalAndLocalProjectsFilter = new BrowseProjectFilter();
//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.

var openDlg = new OpenItemDialog
{
  Title = "Select a Feature Class",
  InitialLocation = @"C:\Data",
  MultiSelect = false,
  BrowseFilter = BrowseProjectFilter.GetFilter(ArcGIS.Desktop.Catalog.ItemFilters.GeodatabaseItems_All)
};

//show the browse dialog
bool? ok = openDlg.ShowDialog();
if (!ok.HasValue || openDlg.Items.Count() == 0)
  return;   //nothing selected

await QueuedTask.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 ArcGIS.Core.Data.FeatureClass fc)
      {
        // create a layer 
        var featureLayerParams = new FeatureLayerCreationParams(fc)
        {
          MapMemberIndex = 0
        };
        var layer = LayerFactory.Instance.CreateLayer<FeatureLayer>(featureLayerParams, MapView.Active.Map);

        // continue
      }
    }
  }
});