ViewAViewModel.cs

public class ViewAViewModel : BindableBase
    {
        private NewsService NewsService;

        private NewsAnnouncement newsAnnouncement;
        public NewsAnnouncement NewsAnnouncement
        {
            get { return newsAnnouncement; }
            set
            {
                SetProperty(ref newsAnnouncement, value);
            }
        }
        private AnnouncementDetail _announcement;
        public AnnouncementDetail Announcement
        {
            get { return _announcement; }
            set
            {
                SetProperty(ref _announcement, value);
            }
        }

        public DelegateCommand<long?> SelectArticle { get; set; }
        public ViewAViewModel()
        {
            this.NewsService = new NewsService("http://www.idx.co.id");
            this.NewsAnnouncement = NewsService.NewsAnnouncements(Locale.IdId, 20);

            SelectArticle = new DelegateCommand<long?>(Select, CanSelect);
        }

        private bool CanSelect(long? arg)
        {
            return true;
        }

        private void Select(long? itemid)
        {
            Debug.WriteLine(itemid + "==============>");
            if (itemid.HasValue)
            {
                Announcement = NewsService.Details(itemid.Value);//
                Item singlnews = NewsAnnouncement.Items.SingleOrDefault(d => d.Id == itemid);
            }
            if (Announcement == null)
            {
                MessageBox.Show("Article not found.");
            }
        }

    }