Unity 2018.1 WebGL fails on CommitUpdate using icsharpcode.sharpziplib

I have been working on a WebGL project using Unity 2017. We use ICSharpCode.SharpZipLib.Zip to generate zip-files and upload them to our server. This worked well using the following code:

public string ZipFiles(List<string> files)
{
    // create new zip file
    string zipPath = Application.persistentDataPath + "/upload.zip";

    try
    {
        // prepare
        FileStream fsOut = File.Create(zipPath);
        ZipFile zip = ZipFile.Create(fsOut);

        // fill
        zip.BeginUpdate();

        foreach (string file in files)
        {
            zip.Add(file, Path.GetFileName(file));
        }
        Debug.Log("Zip before commit");
        zip.CommitUpdate();
        Debug.Log("Zip after commit");
        zip.Close();
        Debug.Log("Zip after close");
        fsOut.Close();
        Debug.Log("Zip filestream closed");
    }
    catch (Exception ex)
    {
        Debug.Log("Exception Zip: " + ex);
    }
    // finish
    return zipPath;
}

Now with the update to Unity 2018.1 we also updated to .Net 4.6 - everything is working fine in Editor.

Using the WebGL build the application fails on zip.CommitUpdate();

Error Log only displays:

NotSupportedException: Encoding 437 data could not be found. Make sure you have correct international codeset assembly installed and enabled.
  at System.Text.Encoding.GetEncoding (System.Int32 codepage) [0x00000] in <00000000000000000000000000000000>:0 

(Filename: currently not available on il2cpp Line: -1)

I think this a very useless error log...

The zip-file in the file-system gets uploaded, but is empty. Files are available: one xml and two json files. (Already made a check with File.exists earlier...)

Can anyone help? Thanks!

Finally figured it out!

I cloned the github project SharpZipLib and created the DLL with .Net Standard 2.0 as ClassLibrary. I replaced my existing DLL with the self created DLL.

Then I made little changes to the code: before we were using a FileStream to generate the zip-file. Now we use the ZipFile.Create(string path) variant. Also we use a DiskArchiveStorage refering to the zip-file. ("on disk")

public string ZipFiles(List<string> files)
{
    // create new zip file
    string zipPath = Application.persistentDataPath + "/upload.zip";

    try
    {
        // prepare
        ZipFile zip = ZipFile.Create(zipPath);

        // fill
        DiskArchiveStorage myDisk = new DiskArchiveStorage(zip);
        zip.BeginUpdate(myDisk);

        foreach (string file in files)
        {
            Debug.Log("ZIP: add " + file);
            if (!File.Exists(file))
            {
                Debug.Log("File not found!");
            }
            zip.Add(file, Path.GetFileName(file));
        }
        zip.CommitUpdate();
        zip.Close();
    }
    catch (Exception ex)
    {
        Debug.Log("Exception Zip: " + ex);
    }

    // finish
    return zipPath;
}

Now it is working like before!