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

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.