Generate thumbnail for list attachment

using System;
using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;
using System.Linq;

namespace QuarterlyAward
{
    public partial class Thumbnail : LayoutsPageBase
    {
        /// <summary>
        /// Create thumbnail for list attachment
        /// Query string parameters:
        /// listID: List id
        /// itemID: Item id of list
        /// w: Specified thumbnail width
        /// h: Specified thumbnail height
        /// c: Use cache if cache exist and parameter value is not set
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        protected void Page_Load(object sender, EventArgs e)
        {
            string cacheID = string.Format("{0}#{1}#{2}#{3}",
                Request.QueryString["listID"],
                Request.QueryString["itemID"],
                Request.QueryString["w"],
                Request.QueryString["h"]);
            byte[] thumbnailBinary = null;

            // Cache detection
            if (string.IsNullOrEmpty(Request.QueryString["c"]) && Cache[cacheID] != null)
            {
                thumbnailBinary = Cache[cacheID] as byte[];
                // Output cached thumbnail
                Response.Clear();
                Response.OutputStream.Write(thumbnailBinary, 0, thumbnailBinary.Length);
                Response.ContentType = "image/png";
                Response.End();
                return;
            }

            // Lack of parameter
            if (string.IsNullOrEmpty(Request.QueryString["listID"]) || string.IsNullOrEmpty(Request.QueryString["itemID"]))
            {
                Response.StatusCode = 404;
                return;
            }

            // Validate parameter
            int itemID = -1;
            if (!int.TryParse(Request.QueryString["itemID"], out itemID))
            {
                Response.StatusCode = 404;
                return;
            }

            int width = -1, height = -1;
            int.TryParse(Request.QueryString["w"], out width);
            int.TryParse(Request.QueryString["h"], out height);
            if (width == -1 && height == -1)
            {
                Response.StatusCode = 404;
                return;
            }

            #region Get attachment and generate thumbnail
            using (var web = SPContext.Current.Site.OpenWeb())
            {
                var list = web.Lists[new Guid(Request.QueryString["listID"])];
                var item = list.GetItemById(itemID);

                if (item == null || item.Attachments == null || item.Attachments.Count == 0)
                {
                    Response.StatusCode = 404;
                    return;
                }

                var imageType = new string[] { ".jpg", ".jpeg", ".gif", ".png", ".bmp" };
                // Get attachment folder
                var folder = item.ParentList.RootFolder.SubFolders["Attachments"].SubFolders[item.ID.ToString()];
                // Get image attachment
                var files = folder.Files
                                  .OfType<SPFile>()
                                  .Where(f => imageType.Contains(System.IO.Path.GetExtension(f.ServerRelativeUrl.ToLower())));
                if (!files.Any())
                {
                    Response.StatusCode = 404;
                    return;
                }
                var file = files.First();

                // Open image binary in memory
                var ms = new System.IO.MemoryStream(file.OpenBinary());
                // Load image instance
                var source = System.Drawing.Image.FromStream(ms);
                // Limit width & height
                if (source.Width < width || width < 1) width = source.Width;
                if (source.Height < height || height < 1) height = source.Height;
                var size = adaptProportionalSize(new System.Drawing.Size(width, height), source.Size);

                // Create thumbnail
                var thumbnailImage = source.GetThumbnailImage(size.Width, size.Height, null, IntPtr.Zero);
                // Get thumbnail binary
                ms = new System.IO.MemoryStream();
                thumbnailImage.Save(ms, System.Drawing.Imaging.ImageFormat.Png);
                thumbnailBinary = ms.ToArray();

                // Cache 30 min
                Cache.Add(
                    cacheID,
                    thumbnailBinary,
                    null,
                    System.Web.Caching.Cache.NoAbsoluteExpiration,
                    new TimeSpan(0, 30, 0),
                    System.Web.Caching.CacheItemPriority.High,
                    null);
            }
            #endregion

            // Output thumbnail
            Response.Clear();
            Response.OutputStream.Write(thumbnailBinary, 0, thumbnailBinary.Length);
            Response.ContentType = "image/png";
            Response.End();
        }

        /// <summary>
        /// Caculate appropriate size for thumbnail
        /// </summary>
        /// <param name="szMax"></param>
        /// <param name="szReal"></param>
        /// <returns></returns>
        System.Drawing.Size adaptProportionalSize(System.Drawing.Size szMax, System.Drawing.Size szReal)
        {
            int nWidth;
            int nHeight;
            double sMaxRatio;
            double sRealRatio;

            if (szMax.Width < 1 || szMax.Height < 1 || szReal.Width < 1 || szReal.Height < 1)
                return System.Drawing.Size.Empty;

            sMaxRatio = (double)szMax.Width / (double)szMax.Height;
            sRealRatio = (double)szReal.Width / (double)szReal.Height;

            if (sMaxRatio < sRealRatio)
            {
                nWidth = Math.Min(szMax.Width, szReal.Width);
                nHeight = (int)Math.Round(nWidth / sRealRatio);
            }
            else
            {
                nHeight = Math.Min(szMax.Height, szReal.Height);
                nWidth = (int)Math.Round(nHeight * sRealRatio);
            }

            return new System.Drawing.Size(nWidth, nHeight);
        }
    }
}