Add Stream wrappers for memory and text-based types by ViveliDuCh · Pull Request #126669 · dotnet/runtime

ViveliDuCh · 2026-04-08T22:31:52Z

Introduces standardized stream wrappers for text and memory types as public sealed classes (StringStream, ReadOnlyMemoryStream, WritableMemoryStream, and ReadOnlySequenceStream ) as approved in API review (video).

What's included

StringStream (System.IO, CoreLib): non-seekable, read-only stream that encodes string or ReadOnlyMemory<char> on-the-fly. Encoding is required (no default). Never emits BOMs.
ReadOnlyMemoryStream (System.IO, CoreLib): seekable, read-only stream over ReadOnlyMemory<byte>
WritableMemoryStream (System.IO, CoreLib): seekable, read-write stream over Memory<byte> with fixed capacity
ReadOnlySequenceStream (System.Buffers, System.Memory): seekable, read-only stream over ReadOnlySequence<byte>
Ref assembly updates for System.Runtime and System.Memory
Updated 3 internal call sites (ReadOnlyMemoryContent, XmlPreloadedResolver, JsonXmlDataContract) to use the new types
Conformance and behavioral tests for all 4 types

Implementation notes

All types are public sealed with direct constructors: the review moved away from factory methods on Stream to avoid derived-type IntelliSense noise
StringStream encodes directly into the caller's buffer in Read() rather than using an internal byte buffer
TryGetBuffer returns false and GetBuffer throws on the memory stream types (no publiclyVisible support for now)
ReadOnlyMemoryStream and WritableMemoryStream derive from Stream directly. Deriving from MemoryStream is the approved shape but deferred pending perf assessment of the private-fields constraint

Limitations

The internal shared-source ReadOnlyMemoryStream in Common/ cannot be fully replaced:
- System.Memory.Data multi-targets netstandard2.0 where the new public type doesn't exist.
- System.Net.Http (net11.0 only) can adopt it directly.

Follow-up work

Deriving ReadOnlyMemoryStream / WritableMemoryStream from MemoryStream (follow-up after base class perf assessment)
IBufferWriter<byte> → Stream wrapper (separate proposal per #100434)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

This PR introduces new standardized Stream wrapper types across CoreLib and System.Memory, aligning with the approved API shape from #82801 and updating a few internal call sites to use the new wrappers.

Changes:

Added new public sealed stream wrappers: StringStream, ReadOnlyMemoryStream, WritableMemoryStream (CoreLib) and ReadOnlySequenceStream (System.Memory).
Updated reference assemblies and project files to expose/compile the new APIs.
Added conformance + targeted behavioral tests, and updated select internal consumers to use StringStream.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs	Implements the new `StringStream` API (read-only, non-seekable, on-the-fly encoding).
src/libraries/System.Private.CoreLib/src/System/IO/ReadOnlyMemoryStream.cs	Implements seekable read-only stream over `ReadOnlyMemory<byte>`.
src/libraries/System.Private.CoreLib/src/System/IO/WritableMemoryStream.cs	Implements seekable fixed-capacity read/write stream over `Memory<byte>`.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Wires new CoreLib stream wrapper source files into the build.
src/libraries/System.Private.CoreLib/src/Resources/Strings.resx	Adds a new resource string entry (currently appears unused).
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates public API surface to include the new stream wrapper types (and a small ref signature normalization change).
src/libraries/System.Runtime/tests/System.IO.Tests/System.IO.Tests.csproj	Includes the new test files in the System.IO test project.
src/libraries/System.Runtime/tests/System.IO.Tests/StringStream/StringStreamConformanceTests.cs	Adds conformance coverage for `StringStream` (string + `ReadOnlyMemory<char>` overloads).
src/libraries/System.Runtime/tests/System.IO.Tests/StringStream/StringStreamTests_String.cs	Adds targeted `StringStream(string, Encoding)` behavioral tests.
src/libraries/System.Runtime/tests/System.IO.Tests/StringStream/StringStreamTests_Memory.cs	Adds targeted `StringStream(ReadOnlyMemory<char>, Encoding)` behavioral tests.
src/libraries/System.Runtime/tests/System.IO.Tests/ReadOnlyMemoryStream/ReadOnlyMemoryStreamConformanceTests.cs	Adds conformance coverage for `ReadOnlyMemoryStream`.
src/libraries/System.Runtime/tests/System.IO.Tests/ReadOnlyMemoryStream/ReadOnlyMemoryStreamTests.cs	Adds targeted `ReadOnlyMemoryStream` behavioral tests.
src/libraries/System.Runtime/tests/System.IO.Tests/WritableMemoryStream/WritableMemoryStreamConformanceTests.cs	Adds conformance coverage for `WritableMemoryStream` (with some overridden tests).
src/libraries/System.Runtime/tests/System.IO.Tests/WritableMemoryStream/WritableMemoryStreamTests.cs	Adds targeted `WritableMemoryStream` behavioral tests.
src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs	Implements `ReadOnlySequenceStream` over `ReadOnlySequence<byte>`.
src/libraries/System.Memory/src/System.Memory.csproj	Includes the new `ReadOnlySequenceStream` source file in System.Memory.
src/libraries/System.Memory/src/Resources/Strings.resx	Adds SR strings needed for stream-like exception messages in System.Memory.
src/libraries/System.Memory/ref/System.Memory.cs	Adds `ReadOnlySequenceStream` to the System.Memory ref surface.
src/libraries/System.Memory/tests/System.Memory.Tests.csproj	Adds tests for `ReadOnlySequenceStream` and references StreamConformanceTests.
src/libraries/System.Memory/tests/ReadOnlyBuffer/ReadOnlySequenceStreamTests.cs	Adds targeted behavioral tests for multi-segment `ReadOnlySequenceStream` scenarios.
src/libraries/System.Memory/tests/ReadOnlyBuffer/ReadOnlySequenceStream.ConformanceTests.cs	Adds conformance coverage for `ReadOnlySequenceStream`.
src/libraries/System.Private.Xml/src/System/Xml/Resolvers/XmlPreloadedResolver.cs	Switches internal string-to-stream conversion to `StringStream`.
src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/JsonXmlDataContract.cs	Switches internal string-to-stream conversion to `StringStream`.

Comments suppressed due to low confidence (1)

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/JsonXmlDataContract.cs:38

Variable is still named 'memoryStream' but the implementation is now StringStream (not a MemoryStream). Rename the local to avoid implying the stream is seekable/in-memory-buffered (e.g., 'xmlContentStream').

            Stream memoryStream = new StringStream(xmlContent, Encoding.UTF8);
            object? xmlValue;
            XmlDictionaryReaderQuotas? quotas = ((JsonReaderDelegator)jsonReader).ReaderQuotas;
            if (quotas == null)
            {
                xmlValue = dataContractSerializer.ReadObject(memoryStream);

src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs

...m.Runtime/tests/System.IO.Tests/WritableMemoryStream/WritableMemoryStreamConformanceTests.cs

...aries/System.Runtime/tests/System.IO.Tests/WritableMemoryStream/WritableMemoryStreamTests.cs

src/libraries/System.Runtime/tests/System.IO.Tests/StringStream/StringStreamTests_Memory.cs

...aries/System.Runtime/tests/System.IO.Tests/WritableMemoryStream/WritableMemoryStreamTests.cs

src/libraries/System.Runtime/ref/System.Runtime.cs

src/libraries/System.Private.CoreLib/src/Resources/Strings.resx

src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs

stephentoub · 2026-04-09T01:22:30Z

src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs

+            if (absolutePosition < 0)
+            {
+                throw new IOException(SR.IO_SeekBeforeBegin);
+            }


Position = -1 will throw an ArgumentOutOfRangeException but Seek(-1, SeekOrigin.Begin) will throw an IOException. Is that consistent with what we do in other Stream types?

As per MemoryStream and UnmanagedMemoryStream, it is: In both, Position setter throws ArgumentOutOfRangeException for negatives, while Seek throws IOException.

src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs

src/libraries/System.Private.CoreLib/src/System/IO/ReadOnlyMemoryStream.cs

src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs

stephentoub · 2026-04-09T01:33:32Z

src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs

+    /// <summary>
+    /// Gets the encoding used by this stream.
+    /// </summary>
+    public Encoding Encoding => _encoding;


I'm curious why we decided to expose the Encoding instance. What's the use case for accessing it?

src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs

stephentoub · 2026-04-09T01:37:09Z

src/libraries/System.Memory/ref/System.Memory.cs

+        public override int Read(System.Span<byte> buffer) { throw null; }
+        public override long Seek(long offset, System.IO.SeekOrigin origin) { throw null; }
+        public override void SetLength(long value) { }
+        public override void Write(byte[] buffer, int offset, int count) { }


This ref is only including overrides for the abstract members whereas the other refs later are also including overrides for the virtual members?

Copilot

Pull request overview

Copilot reviewed 23 out of 23 changed files in this pull request and generated 10 comments.

src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs

Copilot · 2026-04-09T08:11:01Z

src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs

+        /// <inheritdoc />
+        public override long Position
+        {
+            get
+            {
+                EnsureNotDisposed();
+                return _positionPastEnd >= 0 ? _positionPastEnd : _sequence.Slice(_sequence.Start, _position).Length;
+            }
+            set


ReadOnlySequenceStream.Position computes the current offset via _sequence.Slice(_sequence.Start, _position).Length each time. On multi-segment sequences this is O(segments) per call and can become very expensive (e.g., SeekOrigin.Current / Position-heavy loops). Consider tracking a numeric position field updated on Read/Seek/Position set, using SequencePosition only as an iterator cursor.

Copilot · 2026-04-09T08:11:01Z

...aries/System.Runtime/tests/System.IO.Tests/WritableMemoryStream/WritableMemoryStreamTests.cs

+            // Both MemoryStream (fixed capacity) and WritableMemoryStream throw NotSupportedException
+            // when trying to expand beyond capacity, just with different messages
+            var exception = Assert.Throws<NotSupportedException>(() =>
+                stream.Write(data, 0, data.Length));
+
+            // Accept either message format: WritableMemoryStream's or MemoryStream's 'SR.NotSupported_MemStreamNotExpandable' message
+            Assert.True(
+                exception.Message.Contains("Cannot expand buffer") ||
+                exception.Message.Contains("not expandable"),
+                $"Unexpected exception message: {exception.Message}");
+        }


These tests assert on English substrings in NotSupportedException.Message ("Cannot expand buffer" / "not expandable"), which is culture- and implementation-dependent and can fail under localization or message updates. Prefer asserting only on the exception type (and, if needed, observable behavior like Position/Length staying unchanged).

Copilot · 2026-04-09T08:11:02Z

...aries/System.Runtime/tests/System.IO.Tests/WritableMemoryStream/WritableMemoryStreamTests.cs

+            // Both MemoryStream (fixed capacity) and WritableMemoryStream throw NotSupportedException
+            var exception = Assert.Throws<NotSupportedException>(() => stream.WriteByte(4));
+
+            // Accept either message format: WritableMemoryStream's or MemoryStream's 'SR.NotSupported_MemStreamNotExpandable' message
+            Assert.True(
+                exception.Message.Contains("Cannot expand buffer") ||
+                exception.Message.Contains("not expandable"),
+                $"Unexpected exception message: {exception.Message}");
+        }


Same issue here: asserting on exception message substrings makes the test brittle across localization and message changes. Prefer asserting on exception type and/or state invariants rather than Message contents.

Copilot · 2026-04-09T08:11:02Z

src/libraries/System.Runtime/tests/System.IO.Tests/System.IO.Tests.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">


The file now starts with a UTF-8 BOM (invisible char before <Project ...>). Most repo .csproj files are BOM-less; this encoding change creates noisy diffs and can trip tooling. Please remove the BOM so the first character is '<'.

Suggested change

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

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

Copilot · 2026-04-09T08:11:02Z

src/libraries/System.Memory/src/System.Memory.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">


This .csproj now has a UTF-8 BOM at the start of the file. Many other csproj files in the repo (e.g., src/libraries/System.Runtime/src/System.Runtime.csproj) are BOM-less; please remove the BOM to keep encoding consistent and avoid noisy diffs.

Suggested change

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

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

Copilot · 2026-04-09T08:11:03Z

src/libraries/System.Memory/tests/System.Memory.Tests.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">


This .csproj now has a UTF-8 BOM at the start of the file. Please remove the BOM to keep encoding consistent with other csproj files in the repo and reduce diff noise.

Suggested change

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

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

Copilot · 2026-04-09T08:11:03Z

src/libraries/System.Private.CoreLib/src/System/IO/ReadOnlyMemoryStream.cs

+/// <para>The stream cannot be written to. <see cref="CanWrite"/> always returns <see langword="false"/>.</para>
+/// <para><see cref="GetBuffer"/> throws and <see cref="TryGetBuffer"/> returns <see langword="false"/>.</para>
+/// </remarks>
+public sealed class ReadOnlyMemoryStream : MemoryStream
+{


PR description notes ReadOnlyMemoryStream/WritableMemoryStream derive directly from Stream and that deriving from MemoryStream was deferred, but the implementation (and ref) currently inherits MemoryStream. Please align the PR description with the shipped API shape, or adjust the implementation if the intent is still Stream-based.

Copilot · 2026-04-09T08:11:03Z

...aries/System.Runtime/tests/System.IO.Tests/ReadOnlyMemoryStream/ReadOnlyMemoryStreamTests.cs

+        [Fact]
+        public void Constructor_DefaultParameters_CreatesPubliclyVisibleStream()
+        {
+            byte[] buffer = new byte[100];
+            Stream stream = new ReadOnlyMemoryStream(new ReadOnlyMemory<byte>(buffer));
+
+            Assert.True(stream.CanRead);
+            Assert.False(stream.CanWrite);
+            Assert.True(stream.CanSeek);
+            Assert.Equal(100, stream.Length);
+            Assert.Equal(0, stream.Position);
+        }


Test name suggests the stream is "publicly visible", but ReadOnlyMemoryStream explicitly disallows GetBuffer/TryGetBuffer per its remarks/API. Consider renaming this test to reflect what it's actually validating (e.g., basic capabilities/length/position) to avoid confusion.

Copilot · 2026-04-09T08:11:04Z

...m.Runtime/tests/System.IO.Tests/ReadOnlyMemoryStream/ReadOnlyMemoryStreamConformanceTests.cs

+        // Write only stream - no write support
+        protected override Task<Stream?> CreateWriteOnlyStreamCore(byte[]? initialData) => Task.FromResult<Stream?>(null);
+
+        // Read only stream - no read/write support


Comment says "Read only stream - no read/write support" on CreateReadWriteStreamCore, but this override is specifically for a read-write stream factory returning null (because the type doesn't support read-write). Consider updating the comment to avoid implying a read-only stream is being created here.

Suggested change

// Read only stream - no read/write support

// Read/write stream - no read/write support

…onvert edge case. Flush strategy update.

ViveliDuCh and others added 3 commits April 8, 2026 13:17

First API proposal implementation

1f1bfe1

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

API replacements in production code

f7a06f0

Implementation update based on latest API Review final consensus

bd5cf98

ViveliDuCh requested review from Copilot and jozkee April 8, 2026 22:31

github-actions bot added the area-System.Memory label Apr 8, 2026

dotnet-policy-service bot assigned ViveliDuCh Apr 8, 2026

ViveliDuCh added the area-System.IO label Apr 8, 2026

Copilot started reviewing on behalf of ViveliDuCh April 8, 2026 22:32 View session

Copilot AI reviewed Apr 8, 2026

View reviewed changes

stephentoub reviewed Apr 9, 2026

View reviewed changes

Copilot AI review requested due to automatic review settings April 9, 2026 08:01

ViveliDuCh force-pushed the stream-wrappers-api branch from 27761ff to b43698b Compare April 9, 2026 08:01

Copilot started reviewing on behalf of ViveliDuCh April 9, 2026 08:03 View session

ViveliDuCh force-pushed the stream-wrappers-api branch from b43698b to 95a2989 Compare April 9, 2026 08:06

Copilot AI reviewed Apr 9, 2026

View reviewed changes

Address PR feedback. MemoryStream base change. Spillover buffer for C…

5e3f98c

…onvert edge case. Flush strategy update.

ViveliDuCh force-pushed the stream-wrappers-api branch from 95a2989 to 5e3f98c Compare April 9, 2026 19:59

		@@ -1,4 +1,4 @@
		<Project Sdk="Microsoft.NET.Sdk">
		<Project Sdk="Microsoft.NET.Sdk">

	// Read only stream - no read/write support
	// Read/write stream - no read/write support

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ViveliDuCh commented Apr 8, 2026

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