public IReadOnlyList<PortalFolder> PortalFolders {get;}

var portal = ArcGISPortalManager.Current.GetActivePortal();
var owner = portal.GetSignOnUsername();
var userContent = await portal.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
}