ZipArchive, 3mf and compression

[Stefan]

Hi there,
I have a very weird problem I just can't wrap my head around and not even sure where to ask this.
Basically I'm writing a converter for CAD models to the .3mf file format.
.3mf files are renamed .zip files that have a fixed folder structure and use .xml for their internal data.

I use the official 3mf examples for testing my code and here is the problem:
If I use the windows UI to select the files and rightclick -> 'send to zip archive' everything works correctly.
If I use c# ZipFile or sharpziplib to zip the files - the .3mf throws an error.
If I use tar.exe to zip the files the Windows 3D-viewer throws an error but other CAD programs can open the file.

Hp has a filechecker to look for errors in 3mf files and it throws:

FATAL_ERROR

Missing required file _rels/.rels

the file is in the archive and is in the right place but for some reason wont be found by the interpreter.

Is there someone that can point me in the right direction?maybe zip files have different was to store path information that is not visible to me as the user?

 

[Stefan]

Here is hp's validator:

https://dev.3mf.hp3d.io/

and attached the .3mf/zip file

 

[jmcilhinney]

I wonder whether the compression level is specific in 3MF files and that's what is varying between these methods.

 

[Stefan]

I have been thinking about that. The doumentation doesn't specify anything about compression levels. I tested the available modes in ZipFile's CompressionLevel (none, fastest, optimal) with no difference.
3mf specification:

Specification - 3MF Consortium

3mf.io

 

[jmcilhinney]

Can you attach a valid file, an invalid file and post the code you wrote that created that invalid file?

 

[Stefan]

Sure,

code is just this

string startPath = tempFolderPath;
string zipPath = Path.Combine(_Path, _Filename);
ZipFile.CreateFromDirectory(startPath, zipPath, CompressionLevel.Fastest, false);

attached the some files zipped by Windows, and zipped by c# ZipFile.
If you rename them to *.3mf you can open them in 3D-Viewer

 

[jmcilhinney]

I can see that the ratio reported by File Explorer is different for a couple of the files so, assuming that they were from the same source, there certainly seems to be a difference in the way they have been compressed. I don't have time to look any deeper right now though.

 

[Stefan]

I attached another pair of examples that, to me, look pretty much identical in compression.
I'm really scratching my head here.

 

[JohnH]

Would the order of entries in the package matter?

3D\3dmodel.model
_rels\.rels
[Content_Types].xml

[Content_Types].xml
_rels/.rels
3D/3dmodel.model

 

[NoUserHere]

You're both wasting your time with this as it's a problem I have experienced myself, and you're unlikely to resolve the issue. Because i was also unable to resolve when I looked into this previously. The answer is on p1/#3. It has something to do with how the files are compressed, and any change to libraries or compressed files after a file was compressed will likely not be able to decompress. The same applies to files compressed with different compression settings will certainly not be able to decompress with alternative decompressors. These libs are very finicky.

 

[JohnH]

I noticed a probably more important difference, the invalid zip has back slashes in entry names, the valid one forward slashes.
Funny thing is when I use ZipFile.CreateFromDirectory the entries get forward slashes. Then I found this:

Mitigation: ZipArchiveEntry.FullName Path Separator - .NET Framework

Learn how the path separator for the ZipArchiveEntry.FullName property has changed starting with apps that target .NET Framework 4.6.1.

docs.microsoft.com

Starting with apps that target the .NET Framework 4.6.1, the path separator used in the ZipArchiveEntry.FullName property has changed from the backslash ("\") used in previous versions of .NET Framework to a forward slash ("/").

Are you using an old .Net Framework version?

 

[NoUserHere]

I wonder if adding :

<runtime>
   <AppContextSwitchOverrides value="Switch.System.IO.Compression.ZipFile.UseBackslash=false" />
</runtime>

To the project file would make any difference?

I can't try it, as I don't have those libs, nor do I have my own zip files handy, but you might be onto something here. How on earth is one to know that this "might" be a requirement if working with Zip files on different framework versions? Feck sake Microsoft...

If this works, you will have made this board the only board on the net to provide a solution for this problem. When I was searching around for answers to this, I never found a solution nor worked out the underline issue.

 

[NoUserHere]

When did they provide the zip compression namespace as part of system io? ?

 

[NoUserHere]

I've always used external libs. It might explain why the other libs never had answers to this long reported issue.

 

[Stefan]

Thanks for your help so far guys.
You'll have to take babysteps with me here.

where do I add the

<runtime>
   <AppContextSwitchOverrides value="Switch.System.IO.Compression.ZipFile.UseBackslash=false" />
</runtime>

in the *.csproj file?
I'm using VS for all of this and targeting .net framework 4.8