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  public static void CompressAndDownload(HttpResponseBase _response, string zipFilename, List<FileCompressInfo> files)
        {

            MemoryStream outputMemStream = new MemoryStream();
            ZipOutputStream zipStream = new ZipOutputStream(outputMemStream);

            zipStream.SetLevel(3); //0-9, 9 being the highest level of compression
            byte[] bytes = null;
            foreach (var file in files)
            {
                var newEntry = new ZipEntry(file.Filename);
                newEntry.DateTime = DateTime.Now;
                zipStream.PutNextEntry(newEntry);
                bytes = file.Bytes;
                MemoryStream inStream = new MemoryStream(bytes);
                StreamUtils.Copy(inStream, zipStream, new byte[4096]);
                inStream.Close();
                zipStream.CloseEntry();
            }
            zipStream.IsStreamOwner = false;    // False stops the Close also Closing the underlying stream.
            zipStream.Close();                  // Must finish the ZipOutputStream before using outputMemStream.
            outputMemStream.Position = 0;

            MemoryStream ms = new MemoryStream(outputMemStream.ToArray());
            var downloadFilename = string.Format("{0}_{1}", System.DateTime.Now.ToString("yyyy-MM-dd"), zipFilename);

            _response.Clear();
            _response.AddHeader("content-disposition", "attachment; filename=" + downloadFilename);
            _response.ContentType = "application/octet-stream";
            _response.Buffer = true;
            ms.WriteTo(_response.OutputStream);
            _response.End();

        }

    }

    public class FileCompressInfo
    {
        public string Filename { get; set; }
        public byte[] Bytes { get; set; }

    }