Update EncodingTable to use ConcurrentDictionary for performance

[Copilot]

Replace Dictionary + ReaderWriterLockSlim with ConcurrentDictionary in System.Text.Encoding.CodePages's EncodingTable.cs, matching the pattern already used in System.Private.CoreLib's EncodingTable. This eliminates lock acquisition on the fast read path — ConcurrentDictionary.TryGetValue is lock-free.

Description

Changes Made

• EncodingTable.cs: Replace 4 Dictionary fields + 1 ReaderWriterLockSlim with 4 ConcurrentDictionary fields using target-typed new() expressions. Simplify GetCodePageItem, GetCodePageFromName, and GetNameFromCodePage from nested EnterUpgradeableReadLock/EnterWriteLock patterns to straightforward TryGetValue/TryAdd. All multi-line if blocks use braces consistently.
• .csproj: Add System.Collections.Concurrent project reference for $(NetCoreAppCurrent) targets.
• Performance validation: Triggered @EgorBot benchmark comparing PR vs base branch on linux_amd and osx_arm64, exercising hot cache-hit paths through CodePagesEncodingProvider.GetEncoding(name) and encoding property accessors.

Testing

• ✅ Build succeeds — 0 warnings, 0 errors across all target frameworks
• ✅ All 1250 existing tests pass with 0 failures
• ⏳ @EgorBot performance benchmark triggered (awaiting results)

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[stephentoub]

@copilot use the performance benchmark skill to validate this change

[Copilot]

@copilot use the performance benchmark skill to validate this change

Running benchmark via @EgorBot. The benchmark exercises the internal EncodingTable cache lookup paths (which changed from Dictionary + ReaderWriterLockSlim to ConcurrentDictionary) through the public CodePagesEncodingProvider API — both the cold (first-call, populates cache) and hot (subsequent calls, cache hit) paths.

@EgorBot -linux_amd -osx_arm64

using System.Text;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[MemoryDiagnoser]
public class Bench
{
    // Encoding names that exercise EncodingTable.GetCodePageFromName -> cache lookup
    private static readonly string[] EncodingNames = new[]
    {
        "shift_jis", "big5", "gb2312", "euc-kr", "iso-8859-2",
        "windows-1252", "windows-1251", "windows-1250", "windows-874", "ibm437"
    };

    private Encoding[] _cachedEncodings = default!;
    private EncodingProvider _provider = default!;

    [GlobalSetup]
    public void Setup()
    {
        Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
        _provider = CodePagesEncodingProvider.Instance;

        // Pre-populate internal caches by resolving each encoding once
        _cachedEncodings = new Encoding[EncodingNames.Length];
        for (int i = 0; i < EncodingNames.Length; i++)
        {
            _cachedEncodings[i] = _provider.GetEncoding(EncodingNames[i])!;
        }
    }

    // Hot path: repeated lookups by name (hits ConcurrentDictionary/Dictionary cache)
    [Benchmark]
    public int GetEncodingByName_Cached()
    {
        int sum = 0;
        for (int i = 0; i < EncodingNames.Length; i++)
        {
            sum += _provider.GetEncoding(EncodingNames[i])!.CodePage;
        }
        return sum;
    }

    // Hot path: access properties that read from name/item caches
    [Benchmark]
    public int AccessEncodingProperties_Cached()
    {
        int sum = 0;
        for (int i = 0; i < _cachedEncodings.Length; i++)
        {
            var enc = _cachedEncodings[i];
            sum += enc.WebName.Length;
            sum += enc.EncodingName.Length;
            sum += enc.WindowsCodePage;
            sum += enc.IsSingleByte ? 1 : 0;
        }
        return sum;
    }

    // Single lookup (hot path, most representative of typical usage)
    [Benchmark]
    public Encoding GetSingleEncoding_Cached()
    {
        return _provider.GetEncoding("shift_jis")!;
    }
}

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[Copilot]

Pull request overview

This PR replaces the locking mechanism in System.Text.Encoding.CodePages's EncodingTable.cs for improved performance. The original code used four Dictionary instances protected by a single ReaderWriterLockSlim, where even reads required acquiring an upgradeable read lock. The replacement uses four ConcurrentDictionary instances, whose TryGetValue is lock-free on the read path (hot path), matching the pattern already established in System.Private.CoreLib's EncodingTable.

Changes:

• Replace Dictionary<> fields + ReaderWriterLockSlim with ConcurrentDictionary<> fields and simplify all three cache-using methods (GetCodePageItem, GetCodePageFromName, GetNameFromCodePage) from nested lock patterns to straightforward TryGetValue/TryAdd calls.
• Add a ProjectReference to System.Collections.Concurrent for NetCoreAppCurrent-compatible targets, which is the correct gating for source-build project references.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
EncodingTable.cs	Replaces Dictionary + ReaderWriterLockSlim with ConcurrentDictionary; removes ~50 lines of boilerplate locking code
System.Text.Encoding.CodePages.csproj	Adds System.Collections.Concurrent project reference for NetCoreAppCurrent-compatible builds

[tarekgh]

/ba-g the WASM build failure is unrelated to the change and should be already addressed by #125014