System.IO.Packaging - net8 - Package.Flush not flushing all data

[bubibubi]

Description

net8.0
Calling Package.Flush does not flush.
Calling Package.Close works.

net48
Works fine

Reproduction Steps

Make console application with 2 Target Frameworks
Running net48 executable works, running net8.0 executable the MyFile.zip is empty

MyProj.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>net48;net8.0</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="System.IO.Packaging" Version="9.0.3" />
  </ItemGroup>

</Project>

Program.cs

using System;
using System.IO;
using System.IO.Packaging;

class Program
{

    public static void Main()
    {
        var ms = new MemoryStream();

        Package package = Package.Open(ms, FileMode.OpenOrCreate);

        Uri partUri = new Uri("/example.txt", UriKind.Relative);

        PackagePart packagePart = package.CreatePart(partUri, "text/plain");

        using (StreamWriter writer = new StreamWriter(packagePart.GetStream(FileMode.Create, FileAccess.Write)))
        {
            writer.Write("Hello, world!");
        }

        // Replacing package.Flush with package.Close() everything works fine
        package.Flush();


        File.WriteAllBytes("MyFile.zip", ms.GetBuffer());

    }
}

Expected behavior

Save the right content without closing the package

Actual behavior

The file is truncated (or empty)

Regression?

It works with net48

Known Workarounds

No response

Configuration

Which version of .NET is the code running on?
net8.0 (it works with net48
What OS and version, and what distro if applicable?
Windows 11
What is the architecture (x64, x86, ARM, ARM64)?
x64
Do you know whether it is specific to that configuration?
It is specific with net8.0 configuration. net48 works*

Other information

No response

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[ericstj]

This is due to the behavior of the ZipArchive implementation on .NETCore. Zip only ever writes the archive information on close.
https://github.com/dotnet/runtime/blob/15872212c29cecc8d82da4548c3060f2614665f7/src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchive.cs#L314C29-L314C38

On .NETFramework System.IO.Packaging had an entirely different implementation which would write the archive on flush:
https://referencesource.microsoft.com/#WindowsBase/Base/MS/Internal/IO/Zip/ZipArchive.cs,348

I think this is a duplicate of #24149

[bubibubi]

Not so sure it's a duplicate. Flush is implemented but it's not working... It's quite different.