Infer System.Text.Json polymorphism from closed type hierarchies by eiriktsarpalis · Pull Request #130808 · dotnet/runtime

eiriktsarpalis · 2026-07-15T18:21:30Z

Summary

add opt-in closed-hierarchy polymorphism inference to JsonSerializerOptions and source-generation options
support reflection and source generation, including generic specialization, metadata-only hierarchies, deterministic discriminators, and explicit-registration precedence
report unsupported inferred registrations through source-generator diagnostics and add broad functional/unit coverage

Testing

rebuilt src/libraries/System.Text.Json/System.Text.Json.slnx with zero warnings and errors
ran all six System.Text.Json test projects: 130,435 tests, zero failures/errors, and 17 expected skips

Note

This pull request description was generated by GitHub Copilot.

…erence Implements feature dotnet#129041: a global InferClosedTypePolymorphism flag on JsonSerializerOptions and JsonSourceGenerationOptionsAttribute that infers polymorphic \ discriminators from compiler-emitted [IsClosedType] metadata when no explicit [JsonDerivedType] set is present. - Reflection: DefaultJsonTypeInfoResolver infers one derived type per closed hierarchy member; throws for inaccessible/un-unifiable derived types. - Source-gen: forward-compat parser plumbing mirrors reflection parity (throwing factory for inaccessible/open-generic, kept entry for unassignable). - Wire flag into options copy-ctor and cache equality comparer. - New diagnostics SYSLIB1240-1242 for derived-type validation. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…d derived types Source-gen surfaces invalid polymorphic configurations through build-time SYSLIB diagnostics (the established design), so the prior approach of baking a non-localized `throw new InvalidOperationException(...)` into the generated metadata was inconsistent and not localizable. Remove the UnresolvedDerivedTypeError mechanism (the dispatch hook, the GenerateForTypeWithUnresolvedPolymorphism emitter method, the PolymorphismOptionsSpec field, and the two non-localized message constants) and restore the build-diagnostic + drop behavior: - Explicit open-generic [JsonDerivedType] that cannot be unified: report SYSLIB1229 and drop the entry (matches the pre-existing base behavior). - Inferred closed-type derived types that are open-generic-unresolvable or less accessible than the base: report SYSLIB1229 / SYSLIB1241 and drop the entry. Track explicit [JsonDerivedType] presence with a dedicated flag so closed-type inference is still suppressed when an explicit registration exists, even if every explicit entry failed to resolve. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The closed-type inference tests forced an explicit DefaultJsonTypeInfoResolver, which fails under the NativeAOT/source-gen test legs where reflection is disabled. Closed-type polymorphism inference is reflection-only, so rely on the ambient default reflection resolver instead and guard each test on JsonSerializer.IsReflectionEnabledByDefault so they no-op when reflection is unavailable. The forced resolver's stated rationale never applied: this file is only compiled into the reflection test project, not the source-gen one. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The header claimed the closed-type fixtures serve both the reflection-based and source-generated serializers, but the fixture assembly is referenced only by System.Text.Json.Tests and the source-gen path does not support closed-type inference. Correct the comment to name the reflection-based tests as the sole consumer. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Exercise both derived types of the basic hierarchy via [Theory], and add tests for collection-element inference, nested closed-type properties alongside regular properties (with payload round-trip), and the unknown-discriminator deserialization failure path. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Align reflection and source-generation validation, generic specialization, diagnostics, metadata inference, and test coverage. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 8967afcb-a170-497d-930a-a69e5ccf8773

azure-pipelines · 2026-07-15T18:22:17Z

Azure Pipelines: Successfully started running 4 pipeline(s). 11 pipeline(s) were filtered out due to trigger conditions. There may be pipelines that require an authorized user to comment /azp run to run.

dotnet-policy-service · 2026-07-15T18:22:56Z

Tagging subscribers to this area: @dotnet/area-system-text-json
See info in area-owners.md if you want to be subscribed.

Copilot

Pull request overview

This PR adds opt-in “closed type hierarchy” polymorphism inference to System.Text.Json for both reflection-based metadata generation and source generation. When enabled, the resolver/source generator reads compiler-emitted closed-hierarchy metadata (via System.Runtime.CompilerServices.IsClosedTypeAttribute) and synthesizes JsonDerivedType registrations with deterministic string discriminators.

Changes:

Introduces JsonSerializerOptions.InferClosedTypePolymorphism and JsonSourceGenerationOptionsAttribute.InferClosedTypePolymorphism (public API) and wires the setting through reflection + source generator pipelines.
Implements closed-hierarchy derived-type discovery (runtime via CustomAttributeData; source-gen via Roslyn light-up/polyfills), plus accessibility validation and new SYSLIB diagnostics for unsupported inferred registrations.
Adds broad tests covering runtime behavior, source-gen behavior, and diagnostics; updates resources/diagnostic documentation.

Reviewed changes

Copilot reviewed 41 out of 41 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/System.Text.Json.Tests.csproj	Includes new shared closed-type inference test file and `IsClosedTypeAttribute` source for tests.
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/CacheTests.cs	Ensures options caching equality/hash includes `InferClosedTypePolymorphism`.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorDiagnosticsTests.cs	Adds diagnostic coverage for inferred-closed-type scenarios and extends open-generic derived-type diagnostics assertions.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/System.Text.Json.SourceGeneration.Tests.targets	Adds shared closed-type inference tests and `IsClosedTypeAttribute` to source-gen test builds.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/PolymorphicTests.cs	Registers additional types used by closed-type inference/open-generic polymorphism scenarios for source-gen contexts.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/PolymorphicTests.ClosedTypeInference.cs	New source-gen runtime-behavior tests for closed-type inference + runtime guard behavior.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.TestLibrary/TestClasses.cs	Adds referenced closed-type hierarchy types and local compiler-attribute polyfills for test library assembly.
src/libraries/System.Text.Json/tests/Common/PolymorphicTests.CustomTypeHierarchies.cs	Adjusts tests and warning suppressions to align with new diagnostics and source-gen vs reflection validation timing.
src/libraries/System.Text.Json/tests/Common/PolymorphicTests.ClosedTypeInference.cs	New shared (reflection + source-gen) functional coverage for closed-type inference across many hierarchy shapes.
src/libraries/System.Text.Json/tests/Common/JsonCreationHandlingTests.Object.cs	Narrows warning suppression for intentionally invalid derived-type configurations.
src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs	Adds a throw helper for inaccessible inferred derived types.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.Helpers.cs	Formatting-only adjustment near polymorphism population callsite.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/DefaultJsonTypeInfoResolver.Helpers.cs	Implements runtime closed-hierarchy inference, discriminator synthesis, and accessibility validation.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.cs	Adds new public option `InferClosedTypePolymorphism` (and copies it in the copy ctor).
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs	Updates options equality/hash to include `_inferClosedTypePolymorphism`.
src/libraries/System.Text.Json/src/System/ReflectionExtensions.cs	Adds `Type.IsAtLeastAsVisibleAs` ported accessibility comparison used to validate inferred registrations.
src/libraries/System.Text.Json/src/Resources/Strings.resx	Adds runtime error string for inaccessible inferred derived types.
src/libraries/System.Text.Json/ref/System.Text.Json.cs	Adds the new public API surface in the ref assembly (`InferClosedTypePolymorphism` properties).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.zh-Hant.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.zh-Hans.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.tr.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.ru.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.pt-BR.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.pl.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.ko.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.ja.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.it.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.fr.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.es.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.de.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.cs.xlf	Adds new source-gen diagnostic resource entries (untranslated placeholders).
src/libraries/System.Text.Json/gen/Resources/Strings.resx	Adds new source-gen diagnostic strings for derived-type support/accessibility/collision.
src/libraries/System.Text.Json/gen/Model/TypeGenerationSpec.cs	Tracks closed-type-without-inferred-metadata marker for runtime guard emission.
src/libraries/System.Text.Json/gen/Model/SourceGenerationOptionsSpec.cs	Adds `InferClosedTypePolymorphism` to parsed source-gen options model.
src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs	Parses new option, performs closed-type inference, and centralizes derived-type validation/diagnostics.
src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.ExceptionMessages.cs	Adds runtime exception text for “runtime-only inference requested but metadata not generated”.
src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs	Emits runtime guard for closed types lacking inferred metadata; writes the new option into generated default options.
src/libraries/System.Text.Json/gen/JsonSourceGenerator.DiagnosticDescriptors.cs	Introduces SYSLIB1240–1242 source-gen diagnostics for polymorphism validation.
src/libraries/System.Text.Json/gen/Helpers/RoslynExtensions.cs	Adds Roslyn light-up/polyfills for closed-type detection/derived-type enumeration and accessibility comparison.
src/libraries/System.Text.Json/Common/JsonSourceGenerationOptionsAttribute.cs	Adds new public source-gen option property `InferClosedTypePolymorphism`.
docs/project/list-of-diagnostics.md	Documents new SYSLIB1240–1249 reservation range and new diagnostic IDs.

Copilot

Pull request overview

Copilot reviewed 42 out of 42 changed files in this pull request and generated 2 comments.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 42 out of 42 changed files in this pull request and generated 2 comments.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 8967afcb-a170-497d-930a-a69e5ccf8773

Copilot

Pull request overview

Copilot reviewed 41 out of 41 changed files in this pull request and generated 1 comment.

tannergooding

LGTM. Only real feedback is that I didn't see any test validating reflection vs source-gen are equivalent, but I don't think we have existing tests for that either

eiriktsarpalis · 2026-07-16T12:25:53Z

Only real feedback is that I didn't see any test validating reflection vs source-gen are equivalent

The test suite targets an abstraction implemented by both reflection and source gen, so this should ensure the two implementations are mostly observationally equivalent.

eiriktsarpalis · 2026-07-16T16:57:42Z

/ba-g test failure is unrelated

eiriktsarpalis and others added 6 commits July 15, 2026 10:39

Copilot AI review requested due to automatic review settings July 15, 2026 18:21

eiriktsarpalis requested a review from jeffhandley as a code owner July 15, 2026 18:21

github-actions Bot added the area-System.Text.Json label Jul 15, 2026

Copilot started reviewing on behalf of eiriktsarpalis July 15, 2026 18:22 View session

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Comment thread docs/project/list-of-diagnostics.md Outdated

eiriktsarpalis mentioned this pull request Jul 15, 2026

[API Proposal]: JsonPolymorphicAttribute.InferClosedTypePolymorphism #130809

Open

Add IsClosedtypeAttribute dynamic dependency

b615594

Copilot AI review requested due to automatic review settings July 16, 2026 08:47

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Comment thread ...Text.Json/src/System/Text/Json/Serialization/Metadata/DefaultJsonTypeInfoResolver.Helpers.cs

Comment thread docs/project/list-of-diagnostics.md Outdated

Potential fix for pull request finding

aa4de01

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Copilot AI review requested due to automatic review settings July 16, 2026 08:57

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Comment thread src/libraries/System.Text.Json/src/System/ReflectionExtensions.cs

Comment thread src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.cs

Handle Mono closed hierarchy attribute arrays

ed978e9

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Comment thread ...ibraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.TestLibrary/TestClasses.cs

DeagleGross approved these changes Jul 16, 2026

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tannergooding approved these changes Jul 16, 2026

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eiriktsarpalis enabled auto-merge (squash) July 16, 2026 14:54

eiriktsarpalis merged commit 6bd973a into dotnet:main Jul 16, 2026
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eiriktsarpalis deleted the eiriktsarpalis-stj-closed-hierarchies branch July 16, 2026 16:57

build-analysis Bot mentioned this pull request Jul 16, 2026

[android] Android.Device_Emulator.JIT.Test failing on emulators with CoreCLR #112633

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dotnet-maestro Bot mentioned this pull request Jul 17, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#7810

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eiriktsarpalis commented Jul 15, 2026

Summary

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