How to decompress deflate stream (not gzip) using SharpZipLib?

Question

I am working on an app on Windows Phone and trying to get data from a web server by HTTP GET method, and the content-encoding field in the response indicates that the data was compressed by deflate algorithm.

I found the useful library SharpZipLib, which can deal with gzip stream with GZipInputStream, but it does not work for deflate stream.

How can I depress the stream using SharpZipLib? Or is there any other way to deal with that?

Thanks a lot!

score 1 · Accepted Answer · answered Mar 25 '12 at 11:50

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Don't know how you can achieve this with SharpZipLib, but you do have another choice and that is DotNetZip that has a DeflateStream all packed and ready to use here!

Furthermore, AFAIK, DotNetZip is Ms-Pl licensed so it can be used in commercial applications, SharpZipLib (GPL) can't!

answered Mar 25 '12 at 11:50

Pedro Lamas

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SharpZipLib for Windows Phone allows for use with commercial applications if the binary form of the library is used. – Richard Szalay Mar 25 '12 at 17:24
Thanks a lot! The DeflateStream from the DotNetZip works perfectly. – cotaku39 Mar 26 '12 at 03:45

score 1 · Answer 2 · answered Mar 25 '12 at 17:31

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Beware. An http server returning the deflate method may deliver one of two different things. Either a zlib-wrapped (RFC 1950) deflate (RFC 1951) stream (which is what the http standard specifies), or a raw deflate (RFC 1951) stream. The latter has been seen from Microsoft servers written by someone who did not carefully read the standard.

You can resolve this by trying it both ways (starting with the right way), or by not providing deflate as an accepted method from your client, only allowing gzip. There is no ambiguity with gzip.

There may be an interface to zlib to permit you to try both ways, or if not, you may need to write your own interface or use zlib directly. In zlib, you can decode a zlib-wrapped stream by starting with inflateInit(strm), or you can decode a raw deflate stream starting with inflateInit2(strm, -15).

answered Mar 25 '12 at 17:31

Mark Adler

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Thanks for your answer, but the remote server doesn't return gzip stream, and I have no idea which version of deflate it is. Anyway I just solved my problem with DotNetZip mentioned above. – cotaku39 Mar 26 '12 at 03:48
It looks like dotnetzip has a DeflateStream class and a ZlibStream class. If you are using DeflateStream, then the server is (incorrectly) returning raw deflate data. If you are using ZlibStream, the server is (correctly) returning zlib-wrapped deflate data. My point is that for your code to be general, it should try zlib, and if that doesn't work, then try deflate. – Mark Adler Mar 26 '12 at 04:30
@MarkAdler, as of .NET 4.5, DeflateStream internally uses zlib. – Annie Nov 27 '12 at 05:36

How to decompress deflate stream (not gzip) using SharpZipLib?

2 Answers2