[WIP] Add span-based Deflate, ZLib and GZip encoder/decoder APIs #123145

iremyux · 2026-01-13T18:46:21Z

This PR introduces new span-based, streamless compression and decompression APIs for Deflate, ZLib, and GZip formats, matching the existing BrotliEncoder/BrotliDecoder pattern.

New APIs

DeflateEncoder / DeflateDecoder
ZLibEncoder / ZLibDecoder
GZipEncoder / GZipDecoder

These classes provide:

Instance-based API for streaming/chunked compression with Compress(), Decompress(), and Flush()
Static one-shot API via TryCompress() and TryDecompress() for simple scenarios
GetMaxCompressedLength() to calculate buffer sizes

Closes #62113
Closes #39327

src/libraries/System.IO.Compression/src/System/IO/Compression/ZlibEncoderOptions.cs

src/libraries/System.IO.Compression/src/System/IO/Compression/ZlibEncoder.cs

src/libraries/System.IO.Compression/ref/System.IO.Compression.cs

src/libraries/System.IO.Compression/src/System/IO/Compression/ZlibDecoder.cs

src/libraries/System.IO.Compression/src/System/IO/Compression/ZlibEncoder.cs

rzikm · 2026-01-16T10:40:13Z

src/libraries/System.IO.Compression/src/System/IO/Compression/ZlibEncoder.cs

+        /// <returns>One of the enumeration values that describes the status with which the operation finished.</returns>
+        public OperationStatus Flush(Span<byte> destination, out int bytesWritten)
+        {
+            return Compress(ReadOnlySpan<byte>.Empty, destination, out _, out bytesWritten, isFinalBlock: false);


does this force writing output (if available), I think this should lead to FlushCode.SyncFlush to the native API

rzikm · 2026-01-16T10:44:14Z

src/libraries/System.IO.Compression/src/System/IO/Compression/ZlibEncoder.cs

+        /// <param name="source">A read-only span of bytes containing the source data to compress.</param>
+        /// <param name="destination">When this method returns, a span of bytes where the compressed data is stored.</param>
+        /// <param name="bytesWritten">When this method returns, the total number of bytes that were written to <paramref name="destination"/>.</param>
+        /// <param name="compressionLevel">A number representing compression level. -1 is default, 0 is no compression, 1 is best speed, 9 is best compression.</param>


We should be more clear which default we mean.

Suggested change

/// <param name="compressionLevel">A number representing compression level. -1 is default, 0 is no compression, 1 is best speed, 9 is best compression.</param>

/// <param name="compressionLevel">A number representing compression level. -1 means implementation default, 0 is no compression, 1 is best speed, 9 is best compression.</param>

…mpressedLength

AraHaan · 2026-01-22T23:44:38Z

src/libraries/System.IO.Compression/src/System/IO/Compression/DeflateEncoder.cs

+                CompressionLevel.Fastest => ZLibNative.CompressionLevel.BestSpeed,
+                CompressionLevel.NoCompression => ZLibNative.CompressionLevel.NoCompression,
+                CompressionLevel.SmallestSize => ZLibNative.CompressionLevel.BestCompression,
+                _ => throw new ArgumentOutOfRangeException(nameof(compressionLevel)),


This would fail on valid native compression levels not covered by the CompressionLevel enum. Instead I think it should check if the value is is < -1 or > 9 to throw out of range instead.

rzikm · 2026-01-23T12:43:39Z

src/libraries/System.IO.Compression/tests/DeflateZLibGZipEncoderDecoderTests.cs

+
+namespace System.IO.Compression
+{
+    public class DeflateEncoderDecoderTests


Now that Zstandard API is merged, you can rebase to main and make use of

runtime/src/libraries/Common/tests/System/IO/Compression/EncoderDecoderTestBase.cs

Line 16 in c9aa605

public abstract class EncoderDecoderTestBase

Look at e.g.

runtime/src/libraries/System.IO.Compression.Brotli/tests/BrotliEncoderDecoderTests.cs

Line 11 in c9aa605

public class BrotliEncoderDecoderTests : EncoderDecoderTestBase

for how it is integrated.

src/libraries/System.IO.Compression/src/System/IO/Compression/GZipEncoder.cs

rzikm · 2026-01-23T12:57:38Z

src/libraries/System.IO.Compression/src/System/IO/Compression/DeflateEncoder.cs

+        /// <summary>
+        /// Internal constructor to specify windowBits for different compression formats.
+        /// </summary>
+        internal DeflateEncoder(CompressionLevel compressionLevel, ZLibCompressionStrategy strategy, int windowBits)


Would it make sense to expose windowBits in the public API for all types? we would need to do some processing since for some value it needs to be shifted or negated to get the appropriate value that zlib-ng recognizes, but I think it would be nice to be provide consistent configuration knobs for all Encoder* types we have

iremyux and others added 3 commits January 12, 2026 20:46

Add Zlib Encoder and Decoder classes

e402958

Add tests

95ac930

Merge branch 'dotnet:main' into 62113-zlib-encoder-decoder

88acc57

iremyux self-assigned this Jan 13, 2026

iremyux added the area-System.IO.Compression label Jan 13, 2026