Improve code coverage for System.Text.Json with targeted unit tests

[Copilot]

• Mark DeepEquals_TooDeepJsonDocument_ThrowsInsufficientExecutionStackException test as [OuterLoop]
• Add 136 tests systematically targeting uncovered code paths
• Final Coverage: 94.27% line, 90.60% branch, 93.40% method (baseline: 93.73%, +0.54% improvement)
• Fix OutOfMemoryException in validation tests
• Convert all JSON string constants to raw string literals (per code review feedback)
• Replace wasteful JsonSerializerOptions creation with JsonSerializerOptions.Default (per code review feedback)
• Revert incorrect line in JsonValueTests to use options variable (per code review feedback)
• Convert additional JSON strings to raw string literals in DomTests.cs (per code review feedback)
• All 49,787 tests passing

Summary

Through systematic iterations, this PR improved System.Text.Json coverage from 93.73% to 94.27% (+0.54%) by adding 136 targeted unit tests covering previously untested API surfaces including JsonSerializer methods with JsonTypeInfo/JsonSerializerContext, multi-segment Utf8JsonReader scenarios, JsonWriter formatting options, JsonNode operations, JsonNamingPolicy variants, and collection serialization.

Remaining gaps to reach 95% (~215 lines) consist primarily of complex Utf8JsonReader edge cases requiring specialized buffer setups, ThrowHelper exception paths, and F# converter infrastructure.

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[stephentoub]

@copilot, please mark the DeepEquals_TooDeepJsonDocument_ThrowsInsufficientExecutionStackException test as [Outerloop] and try again, following my original instructions.

[Copilot]

@copilot, please mark the DeepEquals_TooDeepJsonDocument_ThrowsInsufficientExecutionStackException test as [Outerloop] and try again, following my original instructions.

Done. Marked the test as [OuterLoop] and reverted the other test changes that had incorrect assertions. All tests now pass. Changes committed in edead34.

[stephentoub]

@copilot:

1. Run the existing test suite for System.Text.Json with coverage enabled (/p:Coverage=true) and collect detailed coverage reports.
2. Analyze the reports to identify uncovered code paths, methods, or branches within System.Text.Json.
3. Systematically add new unit tests targeting uncovered or under-tested areas, aiming to bring coverage as close to 100% as possible while maintaining meaningful test quality.
4. Re-run the coverage analysis with the new tests and compare results to verify improvement.
5. Repeat steps 1-4, focusing on any remaining uncovered segments, until coverage is maximized for all practical purposes.

[Copilot]

@copilot:

1. Run the existing test suite for System.Text.Json with coverage enabled (/p:Coverage=true) and collect detailed coverage reports.
   ...

Working on it:

1. ✅ Ran coverage analysis - baseline: 93.73% line, 90.52% branch, 93.02% method
2. ✅ Analyzed reports - identified uncovered SerializeToDocument/Element/Node methods with JsonTypeInfo parameters, and Deserialize from Span methods
3. ✅ Added 12 new unit tests targeting those uncovered methods
4. Currently building and testing to verify improvement

Next steps: Will re-run coverage analysis and continue adding tests for remaining uncovered areas. Changes committed in eace0c3.

[stephentoub]

@copilot, please keep iterating: run coverage, add more tests to fill gaps, re-run coverage, add more tests to fill gaps, etc. Please try to get line and branch coverage to greater than 95%.

[Copilot]

@copilot, please keep iterating: run coverage, add more tests to fill gaps, re-run coverage, add more tests to fill gaps, etc. Please try to get line and branch coverage to greater than 95%.

Continuing iterations:

Current Progress:

• Baseline: 93.73% line, 90.52% branch
• Current: 94.06% line, 90.51% branch (93.21% method)
• Added 40 new tests across 3 commits (40a1e92, 0b52f02)

Tests Added:

• JsonSerializer Deserialize/Serialize with JsonTypeInfo (generic & non-generic)
• JsonSerializer methods with JsonSerializerContext
• Coverage for Document, Element, Node, Span, Stream, Writer, Reader formats

Next Steps to reach >95%:
Working on additional edge cases and error paths. The remaining gaps are mostly in:

• Utf8JsonReader edge cases (153 uncovered lines)
• ThrowHelper methods (62 uncovered lines)
• F# converters (not easily testable)

Continuing to add more tests...

[Copilot]

Pull Request Overview

This PR systematically improves code coverage for the System.Text.Json library by analyzing coverage reports and adding targeted unit tests. The work progressed through 10 iterations, achieving a +0.54% increase in line coverage (from 93.73% to 94.27%), +0.08% in branch coverage, and +0.38% in method coverage through the addition of 136 new unit tests.

Key changes:

• Added comprehensive test coverage for JsonSerializer DOM APIs with JsonTypeInfo and JsonSerializerContext parameters
• Implemented edge case testing for Utf8JsonReader numeric and floating-point Get methods
• Created extensive multi-segment reading tests for Utf8JsonReader boundary conditions
• Added validation tests for Utf8JsonWriter with extreme input sizes
• Expanded testing for JsonNode, JsonArray, JsonObject, and JsonValue types
• Implemented comprehensive JsonNamingPolicy variant tests
• Added tests for immutable collection serialization
• Marked one long-running test with [OuterLoop] attribute to prevent blocking normal test runs

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Utf8JsonWriterTests.cs	Added 15 tests for extreme input validation, raw value writing, minimized/indented formatting, and various data type serialization edge cases
Utf8JsonReaderTests.cs	Added 3 tests for comment handling (Skip and Allow modes) with single-line and multi-line comments
Utf8JsonReaderTests.TryGet.cs	Added 2 comprehensive tests covering numeric boundary values (byte, int, long, etc.) and floating-point edge cases (single, double)
Utf8JsonReaderTests.MultiSegment.cs	Added 9 tests for complex multi-segment scenarios including nested JSON, long strings, escaped sequences, numbers with exponents, literals, deep nesting, and comments across segment boundaries
OptionsTests.cs	Added 8 tests for PreferredObjectCreationHandling, TypeInfoResolverChain, WriteIndented, and DefaultBufferSize properties
JsonElementTests.cs	Added [OuterLoop] attribute to prevent long-running stack overflow test from blocking normal test runs
DomTests.cs	Added 43 tests for JsonSerializer methods with JsonTypeInfo/JsonSerializerContext parameters, covering Document/Element/Node serialization, span/stream/reader deserialization, and async PipeWriter/PipeReader operations
Array.ReadTests.cs	Added 3 tests for large array and nested array deserialization scenarios
JsonValueTests.cs	Added 8 tests for JsonValue.Create methods with various types and JsonTypeInfo parameters
JsonObjectTests.cs	Added 4 tests for DeepEquals, TryGetPropertyValue, and ContainsKey operations
JsonNodeTests.cs	Added 2 tests for ReplaceWith and Replace operations on nodes and properties
JsonArrayTests.cs	Added 3 tests for DeepEquals with nested arrays and Clear operation behavior
JsonNamingPolicyTests.cs	Added new test file with 30 tests covering all 5 naming policy variants (CamelCase, SnakeCaseLower, SnakeCaseUpper, KebabCaseLower, KebabCaseUpper) with 6 test cases each
JsonDocumentTests.cs	Added 6 tests for GetDouble/GetSingle edge cases, JsonDocumentOptions, and Try methods
CollectionTests.Immutable.Write.cs	Added 4 tests for ImmutableStack, ImmutableQueue, ImmutableHashSet, and ImmutableDictionary serialization

[stephentoub]

@copilot, please address the feedback from @eiriktsarpalis

[Copilot]

@copilot, please address the feedback from @eiriktsarpalis

Fixed in commit 0c7a2e5db. Replaced all instances of creating new JsonSerializerOptions with TypeInfoResolver with JsonSerializerOptions.Default to avoid wasteful cache recreation.

[eiriktsarpalis]

More opportunities to use triple-quote strings here.

[stephentoub]

@eiriktsarpalis, copilot won't respond to address your feedback unless you tag it.

[stephentoub]

@copilot