As described at: https://github.com/OfficeDev/Open-XML-SDK/issues/64#issuecomment-228791344
If you have a zip file which contains a single xml file and try to process it with the following code using Mono 4.4.0 then the operation completes, but on the current Master you get a System.NotSupportedException from System.IO.Compression.DeflateStream.get_Length().
public static void Main(string args)
var fileStream = new FileStream("monoTest.zip", FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite);
var zipArchive = new ZipArchive(fileStream, ZipArchiveMode.Update);
var firstStream = zipArchive.Entries.Open();
var xmlReader = System.Xml.XmlReader.Create(firstStream);
Which looks to be because XmlReader is trying to query the length on the stream you give it, which fails because it's a DeflateStream whose length can't be queried.
This test successfully loads the XML data on MS.NET/Windows.