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Reduce allocation in TextWriter.NewLine #121508

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Reduce allocation in TextWriter.NewLine #121508

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b9b3bcf
Reduce allocation in TextWriter.NewLine
stephentoub Nov 10, 2025
7b2a11c
Merge branch 'main' into newline
stephentoub Nov 11, 2025
80f4d8d
Merge branch 'main' into newline
stephentoub Nov 14, 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
}

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

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