How to download multiple files on a single zip built on demand (using c# or javascript)

Question

I'm struggling achieving a solution to download multiple files at once on my web site.

I've tried a client approach but it didn't work because the files I want to open are images and it doesn't work with images (iframe solution).

I've looked to Google's Picasa or Google+ and it would be perfect to have their multidownload solution: they build their zip "on-demand" which means the browser keeps downloading a file, not knowing its final size, but once it achieved 100%, it stops and everything worked smoothly.

I don't do any idea of how to do this. Any ideas?

Tks!

Answer 1

Based on Alexei Levenkov and OP's comments, I've learned that it is possible to begin writing a zip file to a stream without having to completely assemble it before hand. Fortunately, .NET 4.5 provides a built in utility class for exactly this purpose: System.IO.Compression.ZipArchive .

Unfortunately, as described in this question , this class has a few incompatibilities with the HttpResponse.OutputStream which we intend to write to, since HttpResponse.OutputStream is not seekable, whereas ZipArchive requires any stream it writes to implement the Position member for a seekable stream.

There is hope however: svick has posted an answer that diagnoses the issue and provides a way to work around it. The workaround involves simply creating a "go-between" stream, which implements the members ZipArchive requires, and simply forwards whatever is written to it into another stream (ie Response.OutputStream ).

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If you want to create the zip file server side, which I think is easier, you might want to look at the System.IO.Compression.ZipFile class, which provides static methods for creating archives from existing files.

Eg. creating an archive from a directory on the server:

// In ASP.NET, getting the UNC path to the directory which will be zipped
string dirpath = Server.MapPath('~/app/foldertodownload');

// Destination path
string destpath = Server.MapPath('~/public/downloads.zip');

ZipFile.CreateFromDirectory(dirpath, destpath);

Now all you need to do is write this zip file to the response with the appropriate headers.