[Copilot]

Apple's NSString compare:options:range:locale: is non-deterministic for certain Unicode inputs (C1 control chars \u0080–\u009F), causing StartsWithNoMatch_StringComparison and EndsWithNoMatch_StringComparison to fail intermittently on macOS. The existing [ActiveIssue] only covered IsAppleMobile (iOS/tvOS), leaving macOS unguarded.

Changes

• StartsWithNoMatch_StringComparison: Add [ActiveIssue(..., IsOSX)] referencing issue [Apple-mobile] System.Tests.StringTests.[Starts,Ends]WithNoMatch_StringComparison test failure #108832; replace four separate Assert.Equal calls with a foreach loop using Assert.True(expected == actual, message) that captures length, mismatchIndex, comp, char values at the mismatch index, and the expected/actual boolean values for easier debugging when re-enabled.

• EndsWithNoMatch_StringComparison: Same fix — same root cause (same Apple native API path), same [ActiveIssue] gap, same diagnostic improvement applied consistently.

The diagnostic message format:

String.StartsWith: length=3, mismatchIndex=1, comp=CurrentCulture, chars=(2,3), expected=True, actual=False
Span.StartsWith:   length=3, mismatchIndex=1, comp=CurrentCulture, chars=(2,3), expected=True, actual=False

Original prompt

---

This section details on the original issue you should resolve

<issue_title>OSX failure on StringTests.StartsWithNoMatch_StringComparison</issue_title>
<issue_description>## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=940013
Build error leg or test failing: System.Tests.StringTests.StartsWithNoMatch_StringComparison
Pull request: #111877

Error Message

Fill the error message using step by step known issues guidance.

{
  "ErrorMessage": "[FAIL] System.Tests.StringTests.StartsWithNoMatch_StringComparison",
  "ErrorPattern": "",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Log

[17:27:34] dbug: 2025-02-04 17:27:34.201 Df System.Runtime.Tests[12401:3ab5fc] Failed tests:
[17:27:34] dbug: 2025-02-04 17:27:34.201 Df System.Runtime.Tests[12401:3ab5fc] 1) 	[FAIL] System.Tests.StringTests.StartsWithNoMatch_StringComparison   Test name: System.Tests.StringTests.StartsWithNoMatch_StringComparison
[17:27:34] dbug: Assembly:  [System.Runtime.Tests, Version=10.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51]
[17:27:34] dbug: 2025-02-04 17:27:34.201 Df System.Runtime.Tests[12401:3ab5fc]    Exception messages: Assert.Equal() Failure: Values differ
[17:27:34] dbug: Expected: True
[17:27:34] dbug: Actual:   False   Exception stack traces:    at System.Tests.StringTests.StartsWithNoMatch_StringComparison()

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=940013
Error message validated: [[FAIL] System.Tests.StringTests.StartsWithNoMatch_StringComparison]
Result validation: ❌ Known issue did not match with the provided build.
Validation performed at: 2/5/2025 4:19:32 PM UTC

Report

Build	Definition	Test	Pull Request
1316114	dotnet/runtime	System.Runtime.Tests.WorkItemExecution	#125056

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
0	0	1

Comments on the Issue (you are @copilot in this section)

@tarekgh @matouskozak please triage. Thanks! @matouskozak Looks like duplicate of https://github.com//issues/108832 but with slightly different error message. fyi: @kotlarmilos @danmoseley > [!NOTE] > This analysis was generated by GitHub Copilot.

Analysis: Root cause identifiable from code inspection

A dump or repro isn't strictly necessary here — this is a wrong-return-value bug, not a crash, so a dump wouldn't add much. The root cause is identifiable from the code.

What the test does

The test (StartsWithNoMatch_StringComparison) creates strings from chars \u0001–\u0096 (control characters + basic Latin), introduces a single-char mismatch at each position, then asserts that culture-aware StartsWith returns consistent results between calls:

Assert.Equal(
    firstSpan.ToString().StartsWith(secondSpan.ToString(), StringComparison.CurrentCulture),
    firstSpan.StartsWith(secondSpan, StringComparison.CurrentCulture));

The failure (Expected: True, Actual: False) means the same underlying comparison returns different results on different invocations for the same input.

Where it goes wrong

On Apple mobile in hybrid globalization mode, culture-aware StartsWith routes through GlobalizationNative_StartsWithNative in pal_collation.m:

NSString *prefixStrComposed = RemoveWeightlessCharacters(
    prefixString.precomposedStringWithCanonicalMapping);
NSString *sourceStrComposed = RemoveWeightlessCharacters(
    sourceString.precomposedStringWithCanonicalMapping);

int32_t result = (int32_t)[sourceStrComposed compare:prefixStrComposed
                                    ...

</details>



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- Fixes dotnet/runtime#112195

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[Copilot]

Pull request overview

Updates StringTests to avoid intermittent macOS test failures caused by non-deterministic Apple collation behavior for certain Unicode inputs, by expanding existing platform guards and improving failure diagnostics.

Changes:

• Add [ActiveIssue(..., IsOSX)] to EndsWithNoMatch_StringComparison and StartsWithNoMatch_StringComparison to disable these tests on macOS (in addition to existing Apple mobile coverage).
• Refactor repeated culture-comparison assertions into a loop and add richer diagnostic messages when the assertions fail.

[danmoseley]

@copilot address this

[Copilot]

@copilot address this

Done in 47fc187. All four Assert.True messages now include , expected={expected}, actual={actual} so the boolean values are visible in the failure output alongside the existing context (length, mismatchIndex, comp, chars).

[tarekgh]

LGTM. Thanks @danmoseley!