Is there a built-in zip library in .NET 3.5?

If not, what are the popular open source .net zip libraries.

There is no built-in library. There are open-source options.

DotNetZip is one. Simple, easy to use. It has good features: AES Encryption, regular encryption, streams, ZIP64, file comments, Unicode, progress events, more. And it's free and open source.

Here's some sample code.

    // extract all Photoshop files larger than 100mb
    using (ZipFile zip1 = ZipFile.Read(ZipFileName))
    {
        var LargePhotoShopFiles = zip1.SelectEntries("name = *.psd  and size > 100mb");
        foreach (ZipEntry e in LargePhotoShopFiles)
        {
            if (e.UsesEncryption)
                e.ExtractWithPassword("unpackDirectory", "VerySecret!");
            else 
                e.Extract("unpackDirectory");
        }
    }

EDIT: See note in comments - SharpZipLib is now unmaintained, and you probably want to avoid it.

Open source: #ZipLib

I believe that the classes in the System.IO.Compression namespace are fine for compressing/decompressing a single stream of data, but there's nothing built into the framework to cope with actual zip files.

EDIT: As noted in Ants' answer, there's System.IO.Packaging.ZipPackage but it certainly looks like that's really designed for use in WPF, and wouldn't be terribly convenient to use for general zip file handling. Worth looking into though. I wasn't aware of it before though... definitely worth investigating.

Check out System.IO.Packaging.ZipPackage class.

7Zip will help and its available in multiple languages

http://www.icsharpcode.net/OpenSource/SharpZipLib/

Try System.IO.Compression.DeflateStream.

I will be second to recommend http://www.7-zip.org/sdk.html LZMA SDK, but it's not ZIP.