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Reduce allocation in TextWriter.NewLine #121508

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Reduce allocation in TextWriter.NewLine #121508

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Reduce allocation in TextWriter.NewLine
stephentoub Nov 10, 2025
7b2a11c
Merge branch 'main' into newline
stephentoub Nov 11, 2025
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17 changes: 15 additions & 2 deletions src/libraries/System.Private.CoreLib/src/System/IO/TextWriter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@
// The .NET Foundation licenses this file to you under the MIT license.

using System.Buffers;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.Runtime.CompilerServices;
Expand All @@ -25,6 +26,9 @@ public abstract partial class TextWriter : MarshalByRefObject, IDisposable, IAsy
// We don't want to allocate on every TextWriter creation, so cache the char array.
private static readonly char[] s_coreNewLine = Environment.NewLineConst.ToCharArray();

// s_coreNewLine will be ['\r', '\n'] on Windows and ['\n'] on Unix. This is the opposite, lazily created/cached.
private static char[]? s_otherCoreNewLine;

/// <summary>
/// This is the 'NewLine' property expressed as a char[].
/// It is exposed to subclasses as a protected field for read-only
Expand Down Expand Up @@ -112,8 +116,17 @@ public virtual string NewLine
{
value ??= Environment.NewLineConst;

CoreNewLineStr = value;
CoreNewLine = value.ToCharArray();
if (CoreNewLineStr != value)
{
CoreNewLineStr = value;
CoreNewLine =
value == Environment.NewLineConst ? s_coreNewLine : // current OS default
Environment.NewLineConst == "\r\n" && value == "\n" ? (s_otherCoreNewLine ??= ['\n']) : // other OS default
Environment.NewLineConst == "\n" && value == "\r\n" ? (s_otherCoreNewLine ??= ['\r', '\n']) : // other OS default
value.ToCharArray(); // unknown
Comment on lines +123 to +126
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@GerardSmit GerardSmit Nov 10, 2025 •
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Maybe a switch expression to improve readability?

Suggested change
value == Environment.NewLineConst ? s_coreNewLine : // current OS default
Environment.NewLineConst == "\r\n" && value == "\n" ? (s_otherCoreNewLine ??= ['\n']) : // other OS default
Environment.NewLineConst == "\n" && value == "\r\n" ? (s_otherCoreNewLine ??= ['\r', '\n']) : // other OS default
value.ToCharArray(); // unknown
CoreNewLine = (value, Environment.NewLineConst) switch
{
// current OS default
(Environment.NewLineConst, Environment.NewLineConst) => s_coreNewLine,
// other OS default
("\n", "\r\n") => s_otherCoreNewLine ??= ['\n'],
("\r\n", "\n") => s_otherCoreNewLine ??= ['\r', '\n'],
// unknown
_ => value.ToCharArray(),
};

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@stephentoub stephentoub Nov 10, 2025

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The way I wrote it, the C# compiler will detect that some of the branches are dead and dead-code eliminate them, e.g. on Linux, the line that starts with Environment.NewLineConst == "\r\n" gets entirely removed by the C# compiler (because it sees "\n" == "\r\n"). That doesn't currently happen with the switch variant.

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@GerardSmit GerardSmit Nov 10, 2025 •
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Ah, I see. With tuples Roslyn doesn't see that the branche is dead code.

With the following it does Sharplab / DiffChecker.

Suggested change
value == Environment.NewLineConst ? s_coreNewLine : // current OS default
Environment.NewLineConst == "\r\n" && value == "\n" ? (s_otherCoreNewLine ??= ['\n']) : // other OS default
Environment.NewLineConst == "\n" && value == "\r\n" ? (s_otherCoreNewLine ??= ['\r', '\n']) : // other OS default
value.ToCharArray(); // unknown
CoreNewLine = value switch
{
// other OS default
"\n" when Environment.NewLineConst == "\r\n" => s_otherCoreNewLine ??= ['\n'],
"\r\n" when Environment.NewLineConst == "\n" => s_otherCoreNewLine ??= ['\r', '\n'],
// current OS default
Environment.NewLineConst => s_coreNewLine,
// unknown
_ => value.ToCharArray(),
};

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@stephentoub stephentoub Nov 10, 2025

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Yup, though I personally don't think that improves readability.

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@EgorBo EgorBo Nov 10, 2025

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That doesn't currently happen with the switch variant.

File an issue?

}

Debug.Assert(CoreNewLineStr == new string(CoreNewLine));
}
}

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