According to MSDN in .Net 4.5 System.IO.Compression is based on zlib.
I am trying now to change my current interop based reading from a zlib deflated stream from a non .
What are the first several bytes of the data you are trying to compress?
You might have zlib, gzip, or raw deflate data that you're trying to decode.
By the way, I highly recommend that you use DotNetZip's interface to zlib instead of NET 4.5's (or NET any version). NET 4.5 has bugs in that interface that Microsoft has declared that they won't fix (!).