using System;
using System.Collections.Generic;
using System.Drawing;
using System.IO;
using System.Net;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private static Dictionary<Uri, Image> cache = new Dictionary<Uri, Image>();

        private static Task<Image> DownloadImageAsync(Uri uri)
        {
            var tcs = new TaskCompletionSource<Image>();
            Image cachedImage;
            lock (cache)
            {
                if (cache.TryGetValue(uri, out cachedImage))
                {
                    tcs.SetResult(cachedImage);
                    return tcs.Task;
                }
            }

            var wc = new WebClient();
            wc.DownloadDataCompleted += (s, e) =>
            {
                if (e.Cancelled)
                {
                    tcs.SetCanceled();
                }
                else if (e.Error != null)
                {
                    tcs.SetException(e.Error);
                }
                else
                {
                    Bitmap bmp;
                    using (var stream = new MemoryStream(e.Result))
                    {
                        bmp = new Bitmap(stream);
                    }

                    lock (cache)
                    {
                        if (!cache.ContainsKey(uri))
                        {
                            cache.Add(uri, bmp);
                        }
                    }
                    tcs.SetResult(bmp);
                }
            };

            wc.DownloadDataAsync(uri);
            return tcs.Task;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            var uri = new Uri("http://hoge/sample.png");
            DownloadImageAsync(uri).ContinueWith(t =>
            {
                pictureBox1.Image = t.Result;
            });
        }
    }
}