Change ZipBlocks structs to classes

[carlossanlop]

Contributes to #1541

Structs cannot be passed around in async code (a copy gets created), we need to convert them to classes so they can be used in the incoming async code.

Here's the perf impact comparison before and after:

Method	Toolchain	Mean	Error	StdDev	Median	Min	Max	Ratio	RatioSD	Allocated	Alloc Ratio
ZipFileCreateFromDirectory	Before (struct)	11.16 ms	0.485 ms	0.558 ms	11.13 ms	10.05 ms	12.16 ms	1.00	0.07	10.12 KB	1.00
ZipFileCreateFromDirectory	After (class)	11.59 ms	0.535 ms	0.572 ms	11.41 ms	11.00 ms	13.05 ms	1.04	0.07	10.3 KB	1.02

I used the benchmarks that I am proposing in this PR dotnet/performance#4764

cc @edwardneal you might be interested in this change.

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[Copilot]

Pull Request Overview

This PR converts several ZipBlock types from structs to classes to prevent unintended copying in async scenarios. The key changes include:

• Converting multiple structs (e.g. ZipGenericExtraField, Zip64ExtraField, etc.) into sealed classes.
• Changing default initializations from “default” to “new()” with object initializers.
• Adjusting nullability of fields to support class semantics.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.IO.Compression/src/System/IO/Compression/ZipBlocks.cs	Converted various Zip* types from structs to sealed classes, updating initialization and field nullability.
src/libraries/System.IO.Compression/src/System/IO/Compression/ZipBlocks.FieldLengths.cs	Updated type declarations from struct to sealed class for consistency.
src/libraries/System.IO.Compression/src/System/IO/Compression/ZipBlocks.FieldLocations.cs	Converted type declarations from struct to sealed class for consistency.
src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.cs	Updated default instantiation of Zip64ExtraField references to use “new()”.
src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchive.cs	Updated instantiation of ZipCentralDirectoryFileHeader from struct to class.

Comments suppressed due to low confidence (1)

src/libraries/System.IO.Compression/src/System/IO/Compression/ZipBlocks.cs:780

• The lazy initialization in the ArchiveComment getter guarantees a non-null return but may hide cases where the archive comment was expected to be explicitly set. Please confirm that this behavior is intended and that it does not mask potential errors in data handling.

public byte[] ArchiveComment { get { _archiveComment ??= []; return _archiveComment; } }

[stephentoub]

Can you point to an example for why these need to change to support async?

[carlossanlop]

Can you point to an example for why these need to change to support async?

Of course. We had the exact same problem with Tar: #72472

[michaelgsharp]

LGTM

[edwardneal]

Thanks for the mention @carlossanlop. Some of these classes are fairly large and I think we could save allocating a few instances, but I could easily follow up with those separately if we want to keep this PR clear.

[IDisposable]

Looks pretty good

[carlossanlop]

/ba-g failures are timeouts: The job running on agent NetCore-Public 419 ran longer than the maximum time of 240 minutes.