Optimize SKColor hex parsing and add ReadOnlySpan<char> overloads

[mattleibow]

What

Rewrites SKColor.TryParse to parse hex color strings with a manual, allocation-free hex parser instead of byte.TryParse/uint.TryParse + NumberStyles.HexNumber, and adds public ReadOnlySpan<char> overloads for Parse/TryParse.

Why

SKColor.Parse/TryParse sit on hot paths for anyone hydrating colors from XAML/CSS/JSON/theme files. The old implementation:

• called byte.TryParse per channel and uint.TryParse for the 6/8-digit forms (culture + NumberStyles overhead), and
• had a #if split where the net4x/netstandard2.0 path allocated a new string(char, 2) per channel.

Changes

• Unified, allocation-free parser on ReadOnlySpan<char> (Trim().TrimStart('#') + a length switch, with small case-insensitive nibble helpers). Removes the #if split — one implementation for all TFMs.
• New public overloads (ABI-safe additions; the string overloads delegate via .AsSpan(), matching the existing pattern in SKFont/SKTextBlob/etc.):

  public static SKColor Parse (ReadOnlySpan<char> hexString);
  public static bool TryParse (ReadOnlySpan<char> hexString, out SKColor color);

  These let callers parse a color out of a larger buffer without allocating a substring.
• Tests: whitespace trimming, case-insensitivity (ff/FF/fF), null/empty/#, named-color rejection, span/string agreement, slice parsing, invalid/empty span rejection. 35 SKColorTest cases pass, 0 failed.
• Benchmarks (benchmarks/) documenting the work: a strategy shootout (if-chain vs lookup table vs branchless vs switch), the span-vs-string comparison, and the public New-vs-Old numbers.

Interop / native surface

None. This is a pure-managed change with no P/Invoke involved anywhere:

• SKColor is a readonly struct wrapping a single uint — it has no native handle and no SKObject/IntPtr backing.
• Parse/TryParse and the private TryParseNibble/TryParseByte helpers are 100% managed C#: no SkiaApi.*/sk_* calls, no DllImport/LibraryImport, no Marshal, no stackalloc/fixed, no native memory.
• Parsing a color string therefore never crosses into native Skia — the input goes char-by-char straight into the uint.
• No C API (externals/skia/**) or generated bindings (*.generated.cs) are touched, so externals-download is sufficient for local builds and there is no native rebuild requirement.

Behavior

Preserved for every input the old code accepted, with one intentional tightening: whitespace immediately after # in the 6/8-digit forms (e.g. "# 12345") was previously accepted as a side effect of NumberStyles.HexNumber's AllowLeadingWhite, and is now rejected — matching the stricter behavior the 3/4-digit path already had. Parsing stays case-insensitive.

Performance

net10.0, Apple M3 Pro, BenchmarkDotNet — zero allocations, ~1.4–2.3× faster across all shapes:

Input	New	Old	Speedup
#FFF	3.8 ns	8.4 ns	2.2×
#FFFF	4.0 ns	9.4 ns	2.3×
#FFFFFF	4.8 ns	7.0 ns	1.4×
#FFFFFFFF	6.0 ns	8.9 ns	1.5×
#A7A8A9A0 (varying)	5.1 ns	9.3 ns	1.8×
Red (invalid)	1.7 ns	3.7 ns	2.2×

Notes

• Builds clean on all TFMs (net462/netstandard2.0 included). The old #if guard existed only because that path used the byte/uint.TryParse span overloads (netstandard2.1+); this trimming-only span code compiles everywhere.
• The new public overloads will need XML/mdoc documentation entries — left for the docs generation (CI/api-docs).

[github-actions]

📦 Try the packages from this PR

Warning

Do not run these scripts without first reviewing the code in this PR.

Step 1 — Download the packages

bash / macOS / Linux:

curl -fsSL https://raw.githubusercontent.com/mono/SkiaSharp/main/scripts/get-skiasharp-pr.sh | bash -s -- 4345

PowerShell / Windows:

iex "& { $(irm https://raw.githubusercontent.com/mono/SkiaSharp/main/scripts/get-skiasharp-pr.ps1) } 4345"

Step 2 — Add the local NuGet source

dotnet nuget add source ~/.skiasharp/hives/pr-4345/packages --name skiasharp-pr-4345

More options

Option	Description
--successful-only / -SuccessfulOnly	Only use successful builds
--force / -Force	Overwrite previously downloaded packages
--list / -List	List available artifacts without downloading
--build-id ID / -BuildId ID	Download from a specific build

Or download manually from Azure Pipelines — look for the nuget artifact on the build for this PR.

Remove the source when you're done:

dotnet nuget remove source skiasharp-pr-4345

[github-actions]

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