如何使用ExpressionMediaPlayer.MediaPlayer打开视频文件时直接定位至某一时间点

Question

各位大侠，我有这样一个需求：实现一个函数，它能够在一个Grid上添加一个Mediaplayer、打开视频文件并直接把画面定格在某一时间点。
我是这样写的：

        private void show_video(Grid parent, string filename, TimeSpan time, int width, int height)
        {
            ExpressionMediaPlayer.MediaPlayer media_player = new ExpressionMediaPlayer.MediaPlayer();
            media_player.Height = height;
            media_player.Width = width;
            ExpressionMediaPlayer.PlaylistItem pli = new ExpressionMediaPlayer.PlaylistItem();
            pli.MediaSource = new Uri(filename);

            ExpressionMediaPlayer.ChapterItem chapter = new ExpressionMediaPlayer.ChapterItem();
            chapter.Position = 9;
            pli.Chapters.Add(chapter);

            media_player.Playlist.Items.Add(pli);

            parent.Children.Add(media_player);

            media_player.StateChanged += new RoutedEventHandler(media_player_StateChanged);

            my_time = time;

            seek_time(time);//如何实现定位时间的功能？ 
        }

到 seek_time(time)之前为止，添加mediaplayer并打开视频文件是没问题的，但是之后定位时间该如何操作呢？