dotnet/runtime

System.IO.Compression: ZipArchive loads entire file in memory on .Dispose

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#1,543 opened on Sep 13, 2016

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 (6 comments) (10 reactions) (0 assignees)C# (5,445 forks)batch import
area-System.IO.Compressionenhancementhelp wantedtenet-performance

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Description

When you open a ZipArchive in Update mode, the entire zip file will be loaded in memory when the .Dispose method is invoked.

This is because .Dipose calls .WriteFile, which:

  • Calls LoadLocalHeaderExtraFieldAndCompressedBytesIfNeeded for all entries, which loads the compressed data into memory for those entries
  • Sets the size of the .zip archive to 0, by calling _archiveStream.SetLength(0);
  • Writes out all entries one by one.

As a result:

  • A lot of memory is used, the compressed data for each entry is loaded into memory
  • A lot of unnecessary disk I/O is performed, because all entries are written out again, even if they were not modified.

An alternative may be to incrementally update the zip archive, and only update entries which changes.

Contributor guide

Research direction
Examine the WriteFile method in ZipArchive to understand why it loads all entries into memory. Investigate whether the stream can be updated incrementally without reading all compressed data, focusing on the SetLength and LoadLocalHeaderExtraFieldAndCompressedBytesIfNeeded calls. Look for existing issues or proposals about incremental updates.
Tech stack
csharp
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C#.NETSystem.IO.Compression
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40