You can also go through your geodatabase using the geodatabase API first. In this case you can add your feature classes like this:
if (MapView.Active?.Map == null) return;
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
using (Geodatabase geodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri(@"C:\Data\FeatureTest\FeatureTest.gdb"))))
{
using (FeatureClass addFeatureClass = geodatabase.OpenDataset<FeatureClass>("TestLines"))
{
LayerFactory.Instance.CreateFeatureLayer(addFeatureClass, MapView.Active.Map, 0, "New FC");
}
}
});