http响应Last-Modified和ETag以及asp.net web api实现

// GET /img/2012031023134652.png
        [HttpGet]
        public HttpResponseMessage Get([FromUri]string filename)
        {
            HttpResponseMessage response = new HttpResponseMessage();  

            MongoFSDirectory fs = new MongoFSDirectory(new MongoFSDirectoryParameters() { ConnectionString = this.ConnectionString });
            MongoGridFSFileInfo fileInfo = fs.GetFileInfo(filename);
            if (fileInfo == null)
            {
                throw new HttpResponseException("The file does not exist.", HttpStatusCode.NotFound);
            }
            string etag = string.Format(""{0}"", fileInfo.MD5);
            var tag = Request.Headers.IfNoneMatch.FirstOrDefault();
            if (Request.Headers.IfModifiedSince.HasValue && tag != null && tag.Tag == etag)
            {
                response.StatusCode = HttpStatusCode.NotModified;
            }
            else
            {
                MemoryStream responseStream = new MemoryStream();
                MongoGridFSStream gfs = fileInfo.OpenRead();

                bool fullContent = true;
                if (this.Request.Headers.Range != null)
                {
                    fullContent = false;
                    // Currently we only support a single range.
                    RangeItemHeaderValue range = this.Request.Headers.Range.Ranges.First();

                    // From specified, so seek to the requested position.
                    if (range.From != null)
                    {
                        gfs.Seek(range.From.Value, SeekOrigin.Begin);

                        // In this case, actually the complete file will be returned.
                        if (range.From == 0 && (range.To == null || range.To >= gfs.Length))
                        {
                            gfs.CopyTo(responseStream);
                            fullContent = true;
                        }
                    }
                    if (range.To != null)
                    {
                        // 10-20, return the range.
                        if (range.From != null)
                        {
                            long? rangeLength = range.To - range.From;
                            int length = (int)Math.Min(rangeLength.Value, gfs.Length - range.From.Value);
                            byte[] buffer = new byte[length];
                            gfs.Read(buffer, 0, length);
                            responseStream.Write(buffer, 0, length);
                        }
                        // -20, return the bytes from beginning to the specified value.
                        else
                        {
                            int length = (int)Math.Min(range.To.Value, gfs.Length);
                            byte[] buffer = new byte[length];
                            gfs.Read(buffer, 0, length);
                            responseStream.Write(buffer, 0, length);
                        }
                    }
                    // No Range.To
                    else
                    {
                        // 10-, return from the specified value to the end of file.
                        if (range.From != null)
                        {
                            if (range.From < gfs.Length)
                            {
                                int length = (int)(gfs.Length - range.From.Value);
                                byte[] buffer = new byte[length];
                                gfs.Read(buffer, 0, length);
                                responseStream.Write(buffer, 0, length);
                            }
                        }
                    }
                }
                // No Range header. Return the complete file.
                else
                {
                    gfs.CopyTo(responseStream);
                }
                gfs.Close();
                responseStream.Position = 0;
                response.StatusCode = fullContent ? HttpStatusCode.OK : HttpStatusCode.PartialContent;
                response.Content = new StreamContent(responseStream);
                response.Content.Headers.ContentType = new MediaTypeHeaderValue(fileInfo.ContentType);
                response.Headers.ETag = new EntityTagHeaderValue(etag);
                response.Headers.CacheControl = new CacheControlHeaderValue();
                response.Headers.CacheControl.Public = true;
                response.Headers.CacheControl.MaxAge = TimeSpan.FromHours(480);
                response.Content.Headers.Expires = DateTimeOffset.Now.AddDays(20);
                response.Content.Headers.LastModified = fileInfo.UploadDate;
            }
            return response;

        }