public class PortalFolder : OnlineItem, ArcGIS.Desktop.Core.IMetadata, System.ComponentModel.INotifyPropertyChanged, System.ICloneable  

{
  var activePortal = ArcGISPortalManager.Current.GetActivePortal();
  var owner = activePortal.GetSignOnUsername();
  var activeUserContent = await activePortal.GetUserContentAsync(owner);
  //Get content for a specific folder (identified by its folder id)
  //var userContent = await portal.GetUserContentAsync(owner, folderId);

  //Get all the folders
  foreach (var pf in userContent.PortalFolders)
  {
    //Do something with the folders

  }
  //Get all the content items
  foreach (var pi in userContent.PortalItems)
  {
    //Do something with the portal items
  }
}