If you wish to use the ZlibStream to compress data while writing, you can create a ZlibStream with CompressionMode.Compress, and a writable output stream. Then call Write() on that ZlibStream, providing uncompressed data as input. The data sent to the output stream will be the compressed form of the data written. If you wish to use the ZlibStream to decompress data while writing, you can create a ZlibStream with CompressionMode.Decompress, and a writable output stream. Then call Write() on that stream, providing previously compressed data. The data sent to the output stream will be the decompressed form of the data written.
A ZlibStream can be used for Read() or Write(), but not both.