Add utf8, utf16, utf32 as accepted Encoding.GetEncoding aliases

[tarekgh]

Summary

Adds the hyphen-less encoding names utf8, utf16, and utf32 as accepted aliases in Encoding.GetEncoding(string), mapping them to code pages 65001, 1200, and 12000 respectively.

Today Encoding.GetEncoding("utf8") throws:

System.ArgumentException: 'utf8' is not a supported encoding name.

These hyphen-less spellings are already accepted by essentially every other major platform (Java, Python, ICU, glibc/iconv, Node.js) and are required labels for UTF-8 by the WHATWG Encoding Standard. .NET is the outlier in rejecting them. This surfaces in practice when a server returns a Content-Type header such as application/json; charset=utf8 and HttpClient fails to decode the body (see #133319).

Change

Only the CoreLib name-to-code-page lookup table is affected (System.Private.CoreLib):

• EncodingData.cs: added utf16, utf32, utf8 to the EncodingNames string in ordinal-sorted position, recomputed EncodingNameIndices, and added the matching CodePagesByName entries.

System.Text.Encoding.CodePages needs no change: it owns only legacy code pages and does not participate in resolving Unicode encoding names.

Notes

• These are input-resolution aliases only. Encoding.GetEncodings() and EncodingInfo.Name/WebName are driven by a separate table (MappedCodePages) and are unchanged, so enumeration returns no new or duplicated entries and still reports the canonical names (utf-8, utf-16, utf-32).
• Matches the existing pattern used for other non-IANA aliases already accepted, such as unicode, latin1, and ascii.

Testing

• Extended EncodingGetEncodingTests so the name-to-code-page and EncodingName tests cover utf8/utf16/utf32.
• Full System.Text.Encoding.Tests suite passes (14,695 tests, 0 failed).

Fixes #133319

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[tarekgh]

CC @kilasuit

[Copilot]

🟡 Changes recommended

Update the generator source so the autogenerated alias changes are not overwritten.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds utf8, utf16, and utf32 aliases to Encoding.GetEncoding(string) with regression tests.

Changes:

• Maps aliases to UTF-8, UTF-16, and UTF-32 code pages.
• Extends encoding lookup tests.
• Requires updating the generator source for the autogenerated lookup file.

File summaries

File	Summary
src/libraries/System.Runtime/tests/System.Text.Encoding.Tests/Encoding/EncodingGetEncodingTests.cs	Tests the new alias mappings.
src/libraries/System.Private.CoreLib/src/System/Text/EncodingData.cs	Adds alias lookup entries and indices; generated source requires corresponding generator updates.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

[MihaZupan]

Thanks

[tarekgh]

/ba-g the failures are unrelated