除非用户向下滚动，否则Xamarin在ListView上的第一个图像不会显示

Question

自定义适配器

class MoviesAdapter : BaseAdapter<Movies>
    {
        private List<Movies> movie_list;
        private Context context;

        public MoviesAdapter(Context context, List<Movies> movie_list)
        {
            this.movie_list = movie_list;
            this.context = context;

        }

        public override Movies this[int position]
        {
            get{ return movie_list[position]; }
        }

        public override int Count
        {
            get{ return movie_list.Count; }
        }

        public override long GetItemId(int position)
        {
            return position;  
        }

        public override View GetView(int position, View convertView, ViewGroup parent)
        {

            ViewHolder holder;
            var row = convertView;

            if(row != null)
            {
                holder = row.Tag as ViewHolder;
            }

            else 
            {
                row = LayoutInflater.From(this.context).Inflate(Resource.Layout.show_times, null);
                holder = new ViewHolder();

                holder.title = row.FindViewById<TextView>(Resource.Id.movie_title);
                holder.rating = row.FindViewById<TextView>(Resource.Id.movie_rating);
                holder.summary = row.FindViewById<TextView>(Resource.Id.movie_description);
                holder.poster = row.FindViewById<ImageView>(Resource.Id.poster);
                row.Tag = holder;

            }

            holder.title.Text = movie_list[position].title;
            holder.rating.Text = movie_list[position].rating;
            holder.summary.Text = movie_list[position].summary;

            if (!String.IsNullOrWhiteSpace(movie_list[position].poster))
            {
                holder.poster.SetImageBitmap(movie_list[position].image);
            }
            else
            {
                holder.poster.SetImageResource(Resource.Drawable.Icon);
            }


            return row;
        }

我的活动：

public class ShowTimes : Activity

    {
        private List<Movies> movie_list = new List<Movies>();

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            SetContentView(Resource.Layout.Activity2);
            string theater_name = Intent.GetStringExtra("theater_name");

            TextView theater = FindViewById<TextView>(Resource.Id.theater_name);
            ListView movies_view = FindViewById<ListView>(Resource.Id.movies);
            theater.Text = theater_name;
            var movies = this.get_movies(theater_name);
            foreach(var movie in movies)
            {
                Movies movie_object = movie.ToObject<Movies>();
                if(movie_object.poster != null)
                {
                    download_image(movie_object);
                }
                movie_list.Add(movie_object);

            }
            MoviesAdapter movie_adpater = new MoviesAdapter(this, movie_list);
            movies_view.Adapter = movie_adpater;
        }



        private JArray get_movies(string theater_name)
        {
            var encode_arg = WebUtility.UrlEncode($"{theater_name}");
            string url = $"http://some_address/api/get_showtimes?theater=" + encode_arg;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);

            request.Method = "GET";

            var api_response = (HttpWebResponse)request.GetResponse();
            string output;

            using (StreamReader response_stream = new StreamReader(api_response.GetResponseStream()))
            {
                output = response_stream.ReadToEnd();
            }

            JObject movies = JObject.Parse(output);
            return (JArray)movies["response"];
        }



        public async void download_image(Movies movie_object)
        {
            using (var webclient = new WebClient())
            {
                var image_bytes = await webclient.DownloadDataTaskAsync(movie_object.poster);
                var bit_map = BitmapFactory.DecodeByteArray(image_bytes, 0, image_bytes.Length);
                movie_object.image = bit_map;
            }
        }

    }
}

这就是它的样子

但在向下滚动图像时，所有图像都会出现，向上滚动时，上图中的上一张图像会显示出来。

最新情况如何解决？

我很困惑，因为。描述就在那里，一切都只是表现得很有趣的图像。