How do I zip on the fly and stream to Response.Output in real time?

Question

I am trying to use the following code: I get a corrupted zip file. Why? The file names seem OK. Perhaps they are not relative names, and that's the problem?

      private void trySharpZipLib(ArrayList filesToInclude)
    {
        // Response header
        Response.Clear();
        Response.ClearHeaders();
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        Response.StatusCode = 200; // http://community.icsharpcode.net/forums/p/6946/20138.aspx
        long zipSize = calculateZipSize(filesToInclude);
        string contentValue = 
            string.Format("attachment; filename=moshe.zip;"
                          ); // + " size={0}", zipSize);
        Response.ContentType = "application/octet-stream"; //"application/zip"; 
        Response.AddHeader("Content-Disposition", contentValue);
        Response.Flush();

        using (ZipOutputStream zipOutputStream = new ZipOutputStream(Response.OutputStream) ) 
        {
            zipOutputStream.SetLevel(0);

            foreach (string f in filesToInclude)
            {
                string filename = Path.Combine(Server.MapPath("."), f);
                using (FileStream fs = File.OpenRead(filename))
                {
                    ZipEntry entry =
                        new ZipEntry(ZipEntry.CleanName(filename))
                            {
                                DateTime = File.GetCreationTime(filename),
                                CompressionMethod = CompressionMethod.Stored,
                                Size = fs.Length
                            };
                    zipOutputStream.PutNextEntry(entry);

                    byte[] buffer = new byte[fs.Length];
                    // write to zipoutStream via buffer. 
                    // The zipoutStream is directly connected to Response.Output (in the constructor)
                    ICSharpCode.SharpZipLib.Core.StreamUtils.Copy(fs, zipOutputStream, buffer); 
                    Response.Flush(); // for immediate response to user
                } // .. using file stream
            }// .. each file
        }
        Response.Flush();
        Response.End();
    }

Answer 1

Boy, that's a lot of code! Your job would be simpler using DotNetZip. Assuming a HTTP 1.1 client, this works:

Response.Clear();
Response.BufferOutput = false;
string archiveName= String.Format("archive-{0}.zip", DateTime.Now.ToString("yyyy-MMM-dd-HHmmss"));
Response.ContentType = "application/zip";
// see http://support.microsoft.com/kb/260519
Response.AddHeader("content-disposition", "attachment; filename=" + archiveName);  
using (ZipFile zip = new ZipFile())
{
    // filesToInclude is a IEnumerable<String> (String[] or List<String> etc)
    zip.AddFiles(filesToInclude, "files");
    zip.Save(Response.OutputStream);
}
// Response.End(); // will throw an exception internally.
// Response.Close(); // Results in 'Failed - Network error' in Chrome.
Response.Flush(); // See https://stackoverflow.com/a/736462/481207
// ...more code here...

If you want to password-encrypt the zip, then before the AddFiles(), insert these lines:

    zip.Password = tbPassword.Text; // optional
    zip.Encryption = EncryptionAlgorithm.WinZipAes256; // optional

If you want a self-extracting archive, then replace zip.Save() with zip.SaveSelfExtractor().

---

Addendum; some people have commented to me that DotNetZip is "no good" because it creates the entire ZIP in memory before streaming it out. This isn't the case. When you call AddFiles, the library creates a list of entries - objects that represent the state of the things to be zipped up. There is no compression or encryption done until the call to Save. If you specify a stream to the Save() call, then all the compressed bytes get streamed directly to the client.

In the SharpZipLib model, it's possible to create an entry, then stream it out, then create another entry, and stream it out, and so on. With DotNetZip your app creates the complete list of entries first, and then streams them all out. Neither approach is necessarily "faster" than the other, though for long lists of files, say 30,000, the time-to-first-byte will be faster with SharpZipLib. On the other hand I would not recommend dynamically creating zip files with 30,000 entries.

---

EDIT

As of DotNetZip v1.9, DotNetZip supports a ZipOutputStream as well. I still think it's simpler to do things the way I've shown here, though.

---

Some people have the case where they have zip content that is "mostly the same" for all users, but there's a couple of files that are different for each one. DotNetZip is good at this, too. You can read in a zip archive from a filesystem file, update a few entries (add a few, remove a few, etc), then save to Response.OutputStream. In this case DotNetZip does not re-compress or re-encrypt any of the entries that you haven't changed. Much faster.

Of course DotNetZip is for any .NET app, not only ASP.NET. So you can save to any stream.

If you want more info, check out the site or post on the dotnetzip forums.