[eiriktsarpalis]

Immutable sorted dictionary deserialization currently accumulates entries in a Dictionary<TKey, TValue> before invoking the immutable factory. This applies equality/hash semantics before the destination's ordering comparer, which can collapse comparison-distinct keys or produce the wrong duplicate-key behavior. It also introduces hash-based accumulation for a comparison-based destination.

Changes

• Add a dedicated ImmutableSortedDictionary converter backed by SortedDictionary<TKey, TValue>. Duplicate detection follows the destination key comparer: allowed duplicates remain last-write-wins; disallowed duplicates throw JsonException, including when the values are equal.
• Preserve equality-based buffering for ordinary immutable dictionaries and leave already-safe sorted dictionary/set buffering unchanged.
• Move reflection-only configuration onto the corresponding sealed converters and remove the four redundant WithReflection subclasses. Source-generated metadata continues to supply constructor/add delegates directly; reflection setup retains its method-level trimming and dynamic-code annotations.
• Add public API regressions for equality/comparison disagreement, forbidden key hashing, alternate key representations, nested values, and small-buffer streaming. Add a source-generated trimming/AOT app covering the affected converter families with reflection disabled and analysis-warning suppression disabled.

Validation

All validation used Windows x64 and the repository's Release runtime/library builds.

Check	Result
.\build.cmd clr+libs -rc Release -lc Release	Passed before and after reflection-wrapper consolidation
.\dotnet.cmd build .\src\libraries\System.Text.Json\src\System.Text.Json.csproj -c Release -p:RuntimeConfiguration=Release	Passed for net12.0, net11.0, net10.0, netstandard2.0, and net462
Full reflection, Roslyn 4.4/3.11 source-generation integration, FSharp, and both generator-unit suites	135,290 cases, zero failures, 17 existing skips; net12.0/net481 as applicable
ILLink command below	All 22 apps published and executed successfully before and after consolidation
NativeAOT command below	Both supported apps published and executed successfully before and after consolidation

.\dotnet.cmd msbuild .\src\libraries\System.Text.Json\tests\System.Text.Json.Tests\TrimmingTests\System.Text.Json.TrimmingTests.proj -p:TestTrimming=true -p:Configuration=Release -p:RuntimeConfiguration=Release -p:TargetOS=windows -p:TargetArchitecture=x64
.\dotnet.cmd msbuild .\src\libraries\System.Text.Json\tests\System.Text.Json.Tests\TrimmingTests\System.Text.Json.TrimmingTests.proj -p:RunNativeAotTestApps=true -p:Configuration=Release -p:RuntimeConfiguration=Release -p:TargetOS=windows -p:TargetArchitecture=x64

Every linker/AOT app returned 100, with no linker/AOT diagnostics. The comparer regressions were also demonstrated failing against the original implementation before applying the fix. Non-Windows execution was not performed.

Performance tradeoff

Tree buffering fixes comparison semantics and removes serializer-introduced key hashing, but adds comparison work. BenchmarkDotNet measurements of the dedicated sorted-converter implementation, before the final reflection-hook-only consolidation, showed the following for 1,024 entries with duplicates allowed:

Keys	Baseline	Sorted buffer	Allocated memory
Integer	157.7 us	183.4 us	212,816 B -> 189,320 B (-11%)
String	731.8 us	1,243.5 us	258,248 B -> 213,896 B (-17%)

The string-key case was approximately 70% slower. This is a correctness/hash-resilience change, not a throughput optimization. The final reflection-only consolidation does not change allocation or insertion logic, but those measurements were not rerun after that cleanup. The changed duplicate-key treatment and exception type are observable behavioral corrections.

Note

This pull request was authored with GitHub Copilot.

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