/// <summary>
        /// 上一曲
        /// </summary>
        private void LastMusic()
        {
            List <MusicItemViewModel> selectedItem = MusicList.Where(i => i.IsSelected == true).ToList();

            if (selectedItem.Count > 0)
            {
                Playing = selectedItem.First();
                int index = MusicList.IndexOf(Playing);
                if (index >= 1)
                {
                    Playing.IsSelected = false;
                    Playing            = MusicList[--index];
                    Playing.IsSelected = true;
                    System.Threading.Thread.Sleep(100);
                    PlayMusic(Playing.Music);
                }
                else
                {
                    Playing.IsSelected = false;
                    Playing            = MusicList.Last();
                    Playing.IsSelected = true;
                    System.Threading.Thread.Sleep(100);
                    PlayMusic(Playing.Music);
                }
                PlayIcon = new BitmapImage(new Uri("/Image/full_pause.png", UriKind.Relative));
            }
        }