ASP.net c# File being used by another process

I create thumbnails with the following page, Thumbnail.ashx:

<%@ WebHandler Language="C#" Class="Thumbnail" %>

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.IO;
using System.Web;

public class Thumbnail : IHttpHandler {

    private int _thumbnailSize = 150;

    public void ProcessRequest(HttpContext context) {
        string photoName = context.Request.QueryString["p"];

        string cachePath = Path.Combine(HttpRuntime.CodegenDir, photoName + ".png");
        if (File.Exists(cachePath)) {
            OutputCacheResponse(context, File.GetLastWriteTime(cachePath));
            context.Response.WriteFile(cachePath);
            return;
        }
        string photoPath = context.Server.MapPath("../uploads/originals/" + photoName);
        Bitmap photo;
        try {
            photo = new Bitmap(photoPath);
        }
        catch (ArgumentException) {
            throw new HttpException(404, "Photo not found.");
        }
        context.Response.ContentType = "image/png";
        int width, height;
        if (photo.Width > photo.Height) {
            width = _thumbnailSize;
            height = photo.Height * _thumbnailSize / photo.Width;
        }
        else {
            width = photo.Width * _thumbnailSize / photo.Height;
            height = _thumbnailSize;
        }
        Bitmap target = new Bitmap(width, height);
        using (Graphics graphics = Graphics.FromImage(target)) {
            graphics.CompositingQuality = CompositingQuality.HighSpeed;
            graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
            graphics.CompositingMode = CompositingMode.SourceCopy;
            graphics.DrawImage(photo, 0, 0, width, height);
            using (MemoryStream memoryStream = new MemoryStream()) {
                target.Save(memoryStream, ImageFormat.Png);
                OutputCacheResponse(context, File.GetLastWriteTime(photoPath));
                using (FileStream diskCacheStream = new 开发者_如何学GoFileStream(cachePath, FileMode.CreateNew)) {
                    memoryStream.WriteTo(diskCacheStream);
                }
                memoryStream.WriteTo(context.Response.OutputStream);
            }
        }
    }

    private static void OutputCacheResponse(HttpContext context, DateTime lastModified) {
        HttpCachePolicy cachePolicy = context.Response.Cache;
        cachePolicy.SetCacheability(HttpCacheability.Public);
        cachePolicy.VaryByParams["p"] = true;
        cachePolicy.SetOmitVaryStar(true);
        cachePolicy.SetExpires(DateTime.Now + TimeSpan.FromDays(365));
        cachePolicy.SetValidUntilExpires(true);
        cachePolicy.SetLastModified(lastModified);
    }

    public bool IsReusable {
        get {
            return true;
        }
    }
}

When I try to delete the picture file physically or through code, I get a 'cannot access file, being used by another process" error.

Is this caused by the caching of the thumbnail? Or is the file not being closed somewhere I can't spot?

Could it be my upload file script is causing this?

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You don't appear to be closing your MemoryStream anywhere? Garbage collection alone won't remove a file lock, would it? Tossing in a memoryStream.Close() at the end of ProcessRequest couldn't hurt.