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Avoid materializing Task/ValueTask in StreamReader/StreamWriter #130689

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1a75dbe
Avoid materializing Task objects in StreamReader
jakobbotsch Jul 14, 2026
76fb30e
Clean up in StreamReader, give StreamWriter the same treatment
jakobbotsch Jul 14, 2026
5d9239c
Merge branch 'main' of github.com:dotnet/runtime into streamreader-av…
jakobbotsch Jul 17, 2026
f64e6ef
Use intrinsic
jakobbotsch Jul 17, 2026
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82 changes: 69 additions & 13 deletions src/libraries/System.Private.CoreLib/src/System/IO/StreamReader.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@
using System.Buffers.Binary;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
Expand Down Expand Up @@ -71,14 +72,22 @@ public class StreamReader : TextReader

// We don't guarantee thread safety on StreamReader, but we should at
// least prevent users from trying to read anything while an Async
// read from the same thread is in progress.
// read from the same thread is in progress. We track this with the
// following fields.
//
// Generally we prefer to use the bool _asyncIOInProgress, but in
// certain cases for async1 this would require introducing a wrapper
// state machine, and in those cases we use the Task _asyncReadTask
// instead.
//
private bool _asyncIOInProgress;
private Task _asyncReadTask = Task.CompletedTask;

private void CheckAsyncTaskInProgress()
{
// We are not locking the access to _asyncReadTask because this is not meant to guarantee thread safety.
// We are not locking this access because this is not meant to guarantee thread safety.
// We are simply trying to deter calling any Read APIs while an async Read from the same thread is in progress.
if (!_asyncReadTask.IsCompleted)
if (_asyncIOInProgress || !_asyncReadTask.IsCompleted)
{
ThrowAsyncIOInProgress();
}
Expand All @@ -88,6 +97,24 @@ private void CheckAsyncTaskInProgress()
private static void ThrowAsyncIOInProgress() =>
throw new InvalidOperationException(SR.InvalidOperation_AsyncIOInProgress);

private ThrowOnReadsScope GuardAgainstOtherReads()
{
return new ThrowOnReadsScope(this);
}

private readonly struct ThrowOnReadsScope : IDisposable
{
private readonly StreamReader _reader;

public ThrowOnReadsScope(StreamReader reader)
{
reader._asyncIOInProgress = true;
_reader = reader;
}

public void Dispose() => _reader._asyncIOInProgress = false;
}

// StreamReader by default will ignore illegal UTF8 characters. We don't want to
// throw here because we want to be able to read ill-formed data without choking.
// The high level goal is to be tolerant of encoding errors when we read and very strict
Expand Down Expand Up @@ -898,14 +925,13 @@ private int ReadBuffer(Span<char> userBuffer, out bool readToUserBuffer)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task<string?> task = ReadLineAsyncInternal(cancellationToken);
_asyncReadTask = task;

return new ValueTask<string?>(task);
return new ValueTask<string?>(ReadLineAsyncInternal(cancellationToken));
}

private async Task<string?> ReadLineAsyncInternal(CancellationToken cancellationToken)
{
using ThrowOnReadsScope _ = GuardAgainstOtherReads();

if (_charPos == _charLen && (await ReadBufferAsync(cancellationToken).ConfigureAwait(false)) == 0)
{
return null;
Expand Down Expand Up @@ -1026,14 +1052,13 @@ public override Task<string> ReadToEndAsync(CancellationToken cancellationToken)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task<string> task = ReadToEndAsyncInternal(cancellationToken);
_asyncReadTask = task;

return task;
return ReadToEndAsyncInternal(cancellationToken);
}

private async Task<string> ReadToEndAsyncInternal(CancellationToken cancellationToken)
{
using ThrowOnReadsScope _ = GuardAgainstOtherReads();

// Call ReadBuffer, then pull data out of charBuffer.
StringBuilder sb = new StringBuilder(_charLen - _charPos);
do
Expand Down Expand Up @@ -1070,10 +1095,20 @@ public override Task<int> ReadAsync(char[] buffer, int index, int count)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

if (RuntimeHelpers.IsRuntimeAsync())
{
return ReadAsyncInternalWithGuard(new Memory<char>(buffer, index, count), CancellationToken.None);
}

Task<int> task = ReadAsyncInternal(new Memory<char>(buffer, index, count), CancellationToken.None).AsTask();
_asyncReadTask = task;

return task;

async Task<int> ReadAsyncInternalWithGuard(Memory<char> buffer, CancellationToken cancellationToken)

@VSadov VSadov Jul 20, 2026 •
edited
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ReadAsyncInternalWithGuard is used in only one place. Perhaps it will be cleaner to just manually "inline" the using instead of making a helper?

@VSadov VSadov Jul 20, 2026

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Actually - maybe it is a better to avoid introducing the helpers and just switch everything to use using/_asyncIOInProgress pattern - regardless async1 or not?

The whole thing with the preexisting pattern and a task attached to a reader for unknown duration seems a bit odd.
Is there a reason to keep it (other than local function helpers)?

@jakobbotsch jakobbotsch Jul 22, 2026 •
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ReadAsyncInternalWithGuard is used in only one place. Perhaps it will be cleaner to just manually "inline" the using instead of making a helper?

The caller isn't async, so we can't do that.

Actually - maybe it is a better to avoid introducing the helpers and just switch everything to use using/_asyncIOInProgress pattern - regardless async1 or not?

The whole thing with the preexisting pattern and a task attached to a reader for unknown duration seems a bit odd. Is there a reason to keep it (other than local function helpers)?

There was a worry in #130872 about regressing async1 callers. So here I followed these rules:

  1. If switching to the bool would require an extra state machine/continuation allocation, then retain the Task-based approach if called outside runtime async
  2. If we can switch to the bool-based approach without incurring the extra layer, then do that for both async1/runtime async

The case where we need the wrapper async method is when the called method has other callers that did not use the "IO in progress" mechanism. I did not want to change the behavior of those.

{
using ThrowOnReadsScope _ = GuardAgainstOtherReads();
return await ReadAsyncInternal(buffer, cancellationToken).ConfigureAwait(false);
}
}

public override ValueTask<int> ReadAsync(Memory<char> buffer, CancellationToken cancellationToken = default)
Expand Down Expand Up @@ -1281,10 +1316,20 @@ public override Task<int> ReadBlockAsync(char[] buffer, int index, int count)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

if (RuntimeHelpers.IsRuntimeAsync())
{
return ReadBlockAsyncWithGuard(buffer, index, count);
}

Task<int> task = base.ReadBlockAsync(buffer, index, count);
_asyncReadTask = task;

return task;

async Task<int> ReadBlockAsyncWithGuard(char[] buffer, int index, int count)
{
using ThrowOnReadsScope _ = GuardAgainstOtherReads();
return await base.ReadBlockAsync(buffer, index, count).ConfigureAwait(false);
}
}

public override ValueTask<int> ReadBlockAsync(Memory<char> buffer, CancellationToken cancellationToken = default)
Expand All @@ -1304,6 +1349,11 @@ public override ValueTask<int> ReadBlockAsync(Memory<char> buffer, CancellationT
return ValueTask.FromCanceled<int>(cancellationToken);
}

if (RuntimeHelpers.IsRuntimeAsync())
{
return ReadBlockAsyncInternalWithGuard(buffer, cancellationToken);
}

ValueTask<int> vt = ReadBlockAsyncInternal(buffer, cancellationToken);
if (vt.IsCompletedSuccessfully)
{
Expand All @@ -1313,6 +1363,12 @@ public override ValueTask<int> ReadBlockAsync(Memory<char> buffer, CancellationT
Task<int> t = vt.AsTask();
_asyncReadTask = t;
return new ValueTask<int>(t);

async ValueTask<int> ReadBlockAsyncInternalWithGuard(Memory<char> buffer, CancellationToken token)
{
using ThrowOnReadsScope _ = GuardAgainstOtherReads();
return await ReadBlockAsyncInternal(buffer, token).ConfigureAwait(false);
}
}

private async ValueTask<int> ReadBufferAsync(CancellationToken cancellationToken)
Expand Down
98 changes: 60 additions & 38 deletions src/libraries/System.Private.CoreLib/src/System/IO/StreamWriter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -43,14 +43,22 @@ public class StreamWriter : TextWriter

// We don't guarantee thread safety on StreamWriter, but we should at
// least prevent users from trying to write anything while an Async
// write from the same thread is in progress.
// write from the same thread is in progress. We track this with the
// following fields.
//
// Generally we prefer to use the bool _asyncIOInProgress, but in
// certain cases for async1 this would require introducing a wrapper
// state machine, and in those cases we use Task _asyncWriteTask
// instead.
//
private bool _asyncIOInProgress;
private Task _asyncWriteTask = Task.CompletedTask;

private void CheckAsyncTaskInProgress()
{
// We are not locking the access to _asyncWriteTask because this is not meant to guarantee thread safety.
// We are not locking this access because this is not meant to guarantee thread safety.
// We are simply trying to deter calling any Write APIs while an async Write from the same thread is in progress.
if (!_asyncWriteTask.IsCompleted)
if (_asyncIOInProgress || !_asyncWriteTask.IsCompleted)
{
ThrowAsyncIOInProgress();
}
Expand All @@ -60,6 +68,24 @@ private void CheckAsyncTaskInProgress()
private static void ThrowAsyncIOInProgress() =>
throw new InvalidOperationException(SR.InvalidOperation_AsyncIOInProgress);

private ThrowOnWritesScope GuardAgainstOtherWrites()
{
return new ThrowOnWritesScope(this);
}

private readonly struct ThrowOnWritesScope : IDisposable
{
private readonly StreamWriter _writer;

public ThrowOnWritesScope(StreamWriter writer)
{
writer._asyncIOInProgress = true;
_writer = writer;
}

public void Dispose() => _writer._asyncIOInProgress = false;
}

// The high level goal is to be tolerant of encoding errors when we read and very strict
// when we write. Hence, default StreamWriter encoding will throw on encoding error.
// Note: when StreamWriter throws on invalid encoding chars (for ex, high surrogate character
Expand Down Expand Up @@ -664,14 +690,13 @@ public override Task WriteAsync(char value)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task task = WriteAsyncInternal(value, appendNewLine: false);
_asyncWriteTask = task;

return task;
return WriteAsyncInternal(value, appendNewLine: false);
}

private async Task WriteAsyncInternal(char value, bool appendNewLine)
{
using ThrowOnWritesScope _ = GuardAgainstOtherWrites();

if (_charPos == _charLen)
{
await FlushAsyncInternal(flushStream: false, flushEncoder: false).ConfigureAwait(false);
Expand Down Expand Up @@ -714,10 +739,7 @@ public override Task WriteAsync(string? value)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task task = WriteAsyncInternal(value.AsMemory(), appendNewLine: false, default);
_asyncWriteTask = task;

return task;
return WriteAsyncInternal(value.AsMemory(), appendNewLine: false, default);
}
else
{
Expand Down Expand Up @@ -748,10 +770,7 @@ public override Task WriteAsync(char[] buffer, int index, int count)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task task = WriteAsyncInternal(new ReadOnlyMemory<char>(buffer, index, count), appendNewLine: false, cancellationToken: default);
_asyncWriteTask = task;

return task;
return WriteAsyncInternal(new ReadOnlyMemory<char>(buffer, index, count), appendNewLine: false, cancellationToken: default);
}

public override Task WriteAsync(ReadOnlyMemory<char> buffer, CancellationToken cancellationToken = default)
Expand All @@ -770,13 +789,13 @@ public override Task WriteAsync(ReadOnlyMemory<char> buffer, CancellationToken c
return Task.FromCanceled(cancellationToken);
}

Task task = WriteAsyncInternal(buffer, appendNewLine: false, cancellationToken: cancellationToken);
_asyncWriteTask = task;
return task;
return WriteAsyncInternal(buffer, appendNewLine: false, cancellationToken: cancellationToken);
}

private async Task WriteAsyncInternal(ReadOnlyMemory<char> source, bool appendNewLine, CancellationToken cancellationToken)
{
using ThrowOnWritesScope _ = GuardAgainstOtherWrites();

int copied = 0;
while (copied < source.Length)
{
Expand Down Expand Up @@ -826,10 +845,7 @@ public override Task WriteLineAsync()
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task task = WriteAsyncInternal(ReadOnlyMemory<char>.Empty, appendNewLine: true, cancellationToken: default);
_asyncWriteTask = task;

return task;
return WriteAsyncInternal(ReadOnlyMemory<char>.Empty, appendNewLine: true, cancellationToken: default);
}

public override Task WriteLineAsync(char value)
Expand All @@ -846,10 +862,7 @@ public override Task WriteLineAsync(char value)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task task = WriteAsyncInternal(value, appendNewLine: true);
_asyncWriteTask = task;

return task;
return WriteAsyncInternal(value, appendNewLine: true);
}

public override Task WriteLineAsync(string? value)
Expand All @@ -871,10 +884,7 @@ public override Task WriteLineAsync(string? value)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task task = WriteAsyncInternal(value.AsMemory(), appendNewLine: true, default);
_asyncWriteTask = task;

return task;
return WriteAsyncInternal(value.AsMemory(), appendNewLine: true, default);
}

public override Task WriteLineAsync(char[] buffer, int index, int count)
Expand All @@ -900,10 +910,7 @@ public override Task WriteLineAsync(char[] buffer, int index, int count)
ThrowIfDisposed();
CheckAsyncTaskInProgress();

Task task = WriteAsyncInternal(new ReadOnlyMemory<char>(buffer, index, count), appendNewLine: true, cancellationToken: default);
_asyncWriteTask = task;

return task;
return WriteAsyncInternal(new ReadOnlyMemory<char>(buffer, index, count), appendNewLine: true, cancellationToken: default);
}

public override Task WriteLineAsync(ReadOnlyMemory<char> buffer, CancellationToken cancellationToken = default)
Expand All @@ -921,10 +928,7 @@ public override Task WriteLineAsync(ReadOnlyMemory<char> buffer, CancellationTok
return Task.FromCanceled(cancellationToken);
}

Task task = WriteAsyncInternal(buffer, appendNewLine: true, cancellationToken: cancellationToken);
_asyncWriteTask = task;

return task;
return WriteAsyncInternal(buffer, appendNewLine: true, cancellationToken: cancellationToken);
}

public override Task FlushAsync()
Expand All @@ -936,6 +940,12 @@ public override Task FlushAsync()

ThrowIfDisposed();
CheckAsyncTaskInProgress();

if (RuntimeHelpers.IsRuntimeAsync())
{
return FlushAsyncInternalWithGuard(flushStream: true, flushEncoder: true, CancellationToken.None);
}

return (_asyncWriteTask = FlushAsyncInternal(flushStream: true, flushEncoder: true, CancellationToken.None));
}

Expand All @@ -956,9 +966,21 @@ public override Task FlushAsync(CancellationToken cancellationToken)

ThrowIfDisposed();
CheckAsyncTaskInProgress();

if (RuntimeHelpers.IsRuntimeAsync())
{
return FlushAsyncInternalWithGuard(flushStream: true, flushEncoder: true, cancellationToken);
}

return (_asyncWriteTask = FlushAsyncInternal(flushStream: true, flushEncoder: true, cancellationToken));
}

private async Task FlushAsyncInternalWithGuard(bool flushStream, bool flushEncoder, CancellationToken cancellationToken)
{
using ThrowOnWritesScope _ = GuardAgainstOtherWrites();
await FlushAsyncInternal(flushStream, flushEncoder, cancellationToken).ConfigureAwait(false);
}

private Task FlushAsyncInternal(bool flushStream, bool flushEncoder, CancellationToken cancellationToken = default)
{
if (cancellationToken.IsCancellationRequested)
Expand Down
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