Support open generics in polymorphic serialization in System.Text.Json by Copilot · Pull Request #127318 · dotnet/runtime

Copilot · 2026-04-23T11:43:04Z

Description

Polymorphic serialization had no mechanism for open generic derived type declarations. This adds support for resolving open generic types in [JsonDerivedType] attributes against the constructed base type's type arguments.

[JsonDerivedType(typeof(Bar<>), "bar")]
public class Foo<T>;
public class Bar<T> : Foo<T>;

// Now works — Bar<> resolves to Bar<int> from Foo<int>
JsonSerializer.Serialize<Foo<int>>(new Bar<int>());
// Output: {"$type":"bar"}

Also supports complex forms like Derived<T> : Base<List<T>>, interfaces, and nested types.

Changes

DefaultJsonTypeInfoResolver.Helpers (reflection-only path): Added ResolveOpenGenericDerivedTypes with TryResolveOpenGenericDerivedType using Type.MakeGenericType. This is called only from the reflection-based resolver, keeping MakeGenericType out of shared code paths that run in Native AOT. Both methods propagate [RequiresUnreferencedCode] and [RequiresDynamicCode] from the caller CreateTypeInfoCore. Null DerivedType values are handled safely via is { IsGenericTypeDefinition: true } pattern matching.
JsonPolymorphismOptions.CreateFromAttributeDeclarations: Skips open generic derived types (IsGenericTypeDefinition). They are resolved separately by the reflection resolver or source generator.
JsonTypeInfo.PopulatePolymorphismMetadata: Now conditionally skips attribute-based resolution when PolymorphismOptions has already been set by the source generator. This preserves binary compatibility with source-generated code from earlier versions of STJ — older source-gen code that doesn't set polymorphism options will still fall back to attribute-based resolution.
PolymorphicTypeResolver: Unchanged — no open generic handling here. Open generics are resolved before they reach the resolver.
Source generator Parser: Added TryResolveOpenGenericDerivedType helper method using Roslyn's type system (INamedTypeSymbol.Construct()) with full type parameter unification. Accepts ITypeSymbol baseType and handles the generic type check internally. Resolves open generics at compile time, enqueues the closed types for generation, and extracts discriminator values from the attribute. Resolved derived types are sorted by FullyQualifiedName for deterministic compilation.
Source generator Emitter: Creates a single JsonPolymorphismOptions instance, adds all resolved derived types, then assigns to jsonTypeInfo.PolymorphismOptions. Uses proper type ref constants (JsonPolymorphismOptionsTypeRef, JsonDerivedTypeTypeRef). No runtime MakeGenericType in the source-gen path.
Source generator Model: New PolymorphicDerivedTypeSpec record to carry resolved open generic derived type data through the incremental pipeline.
Error handling: New Polymorphism_OpenGenericDerivedTypeNotSupported runtime message for unresolvable open generics (non-generic base, unresolvable type params, constraint violations). SYSLIB1227 source generator diagnostic for open generics that cannot be resolved from the base type's generic arguments.
Tests: 13 new tests covering serialization/deserialization round-trips, multiple derived types, nested classes, interface bases, wrapped type args (Derived<T> : Base<List<T>>), programmatic API with closed types, and invalid pattern rejection. Updated DefaultResolver_ReportsCorrectPolymorphismMetadata to handle open generic derived types that are resolved to closed types.

Architecture

The key architectural decision (per reviewer feedback) is that Type.MakeGenericType is never called from shared code paths — only from DefaultJsonTypeInfoResolver.Helpers (reflection-only). The reflection-only methods properly propagate [RequiresUnreferencedCode] and [RequiresDynamicCode] attributes through the call chain (CreateTypeInfoCore → ResolveOpenGenericDerivedTypes → TryResolveOpenGenericDerivedType) rather than using suppressions. The source generator resolves open generics at compile time using Roslyn's type system and emits the concrete types directly, making this fully Native AOT compatible.

For binary compatibility, PopulatePolymorphismMetadata() (called by JsonMetadataServices for source-gen path) conditionally preserves attribute-based resolution: it only reads [JsonDerivedType] attributes if PolymorphismOptions hasn't already been set by the source generator. This ensures older source-generated code (which didn't populate polymorphism options directly) continues to work.

Rejected patterns (throw `InvalidOperationException`)

// Base is not generic — can't resolve T
[JsonDerivedType(typeof(Derived<>))] class Base;
class Derived<T> : Base;

// T is not used in the base relationship — can't resolve
[JsonDerivedType(typeof(Derived<>))] class Base<T>;
class Derived<T> : Base<int>;

- Add TryResolveOpenGenericDerivedType with type unification to PolymorphicTypeResolver - Resolve open generic derived types in PolymorphicTypeResolver constructor - Add new error message for unresolvable open generic derived types - Add SYSLIB1227 source generator diagnostic for invalid open generics - Update existing test to expect success for now-valid generic scenario Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/f39c123c-a576-4d8d-9da4-b02cd81b6577 Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/f39c123c-a576-4d8d-9da4-b02cd81b6577 Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

…ay "open generic type definitions" instead of "generic types" Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/f39c123c-a576-4d8d-9da4-b02cd81b6577 Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

eiriktsarpalis

@copilot

eiriktsarpalis · 2026-04-23T13:06:36Z

            PolymorphicGenericClass<int> value = new PolymorphicGenericClass<int>.DerivedClass();
-            await Assert.ThrowsAsync<InvalidOperationException>(() => Serializer.SerializeWrapper(value));
+            string json = await Serializer.SerializeWrapper(value);
+            Assert.Equal("{}", json);


Please add deserialization validation as well. Also include a few properties in both layers for good measure.

Addressed in 3c4d937. Added BaseValue and DerivedValue properties to both layers. Deserialization validation is included in the tests that have type discriminators (e.g., OpenGenericDerivedType_WithStringDiscriminator_DeserializationWorks, OpenGenericDerivedType_ProgrammaticApi_Works). The no-discriminator PolymorphicGenericClass test only validates serialization since without a discriminator, deserialization can't distinguish the derived type.

eiriktsarpalis · 2026-04-23T16:17:16Z

 |  __`SYSLIB1225`__ | Type includes ref like property, field or constructor parameter. |
 |  __`SYSLIB1226`__ | 'JsonIgnoreCondition.Always' is not valid on type-level 'JsonIgnoreAttribute' annotations. |
-|  __`SYSLIB1227`__ | _`SYSLIB1220`-`SYSLIB1229` reserved for System.Text.Json.SourceGeneration._ |
+|  __`SYSLIB1227`__ | Open generic derived type is not supported on a non-generic base type. |


The diagnostic description is slightly more narrow than what it is actually doing, i.e. the base type may be generic but the derived type generics are not resolvable to it.

Updated in 3c4d937. The SYSLIB1227 diagnostic now reads: "Open generic derived type is not resolvable for the polymorphic base type" / "The type arguments of the open generic derived type must be resolvable from the base type's generic arguments." This covers both non-generic base and unresolvable type argument cases.

eiriktsarpalis · 2026-04-23T16:18:55Z

                        var derivedType = (ITypeSymbol)attributeData.ConstructorArguments[0].Value!;
-                        EnqueueType(derivedType, typeToGenerate.Mode);
+
+                        // Open generic derived types (e.g. typeof(Derived<>)) are resolved at runtime


We should NEVER do any open generic resolution at runtime, since that is typically not supported in native AOT. It is the source generator's job to do this, and continue type graph traversal in the reconstituted specialization.

Addressed in 3c4d937. The source generator now resolves open generics at compile time using Roslyn's type system and emits the resolved closed types directly. The parser uses TryResolveOpenGenericDerivedType (Roslyn-based type unification) and the emitter generates PolymorphismOptions.DerivedTypes.Add(new JsonDerivedType(typeof(Resolved<int>), "disc")) after CreateObjectInfo. No runtime resolution occurs in the source-gen path.

…dd source generator compile-time resolution - Revert PolymorphicTypeResolver to original (no open generic handling) - Move open generic resolution to DefaultJsonTypeInfoResolver.Helpers (reflection-only) - Add TryResolveOpenGenericDerivedType with Roslyn type unification in source generator Parser - Source generator emits resolved derived types directly into PolymorphismOptions - Add PolymorphicDerivedTypeSpec model record for source generator - CreateFromAttributeDeclarations skips open generic derived types (null-safe) - Update diagnostic description to cover broader unresolvable cases - Fix tests: remove deserialization from no-discriminator test, use closed types in programmatic API test Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/ad0de94b-b94b-48dd-8df5-4159bc77a3c0 Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

jkotas · 2026-04-23T17:24:55Z

+            }
+        }
+
+        [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2055:UnrecognizedReflectionPattern",


@copilot These suppressions are invalid. Propagate the requirement using RequiresUnreferencedCode and RequiresDynamicCode attributes instead

Addressed in ce67a92. Removed the three UnconditionalSuppressMessage attributes and added [RequiresUnreferencedCode] and [RequiresDynamicCode] to both TryResolveOpenGenericDerivedType and its caller ResolveOpenGenericDerivedTypes. The caller CreateTypeInfoCore already has both attributes so the chain is fully propagated.

@copilot no annotations should be necessary, the containing type is already annotated RequiresUnreferencedCode and RequiresDynamicCode.

DefaultJsonTypeInfoResolver doesn't have type-level [RequiresUnreferencedCode]/[RequiresDynamicCode] — only individual methods do. Removing the annotations from these methods causes build failures (IL2055 for MakeGenericType, IL2070 for GetInterfaces, IL3050 for AOT). The current approach matches the existing pattern in this file where every private method that uses reflection has both attributes (e.g., CreateTypeInfoCore, AddMembersDeclaredBySuperType, DetermineCreateObjectDelegate, etc.).

@eiriktsarpalis The type annotations do not apply to static methods. I expect that copilot is going to find they are necessary.

…quiresDynamicCode propagation Remove invalid suppressions on TryResolveOpenGenericDerivedType and propagate the requirements using RequiresUnreferencedCode and RequiresDynamicCode on both ResolveOpenGenericDerivedTypes and TryResolveOpenGenericDerivedType. The caller CreateTypeInfoCore already has both attributes. Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/d2df80f8-8ef2-4e50-8949-b80c4ab36d89 Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

eiriktsarpalis

@copilot please find the location that handles source generated JsonTypeInfo and currently resolved derived type metadata using attribute resolution. This is no longer necessary given that the source generator does this for us, however in order to preserve binary compatibility with source generated code from earlier versions of STJ, we need to conditionally preserve the attribute logic. This can be triggered if we detect that no polymorphism metadata is already present on the JsonTypeInfo.

eiriktsarpalis · 2026-04-23T18:34:09Z

+                            int i => $", {i}",
+                            _ => "",
+                        };
+                        writer.WriteLine($"({JsonTypeInfoLocalVariableName}.{PolymorphismOptionsPropName} ??= new()).DerivedTypes.Add(new global::System.Text.Json.Serialization.Metadata.JsonDerivedType(typeof({derivedTypeSpec.DerivedType.FullyQualifiedName}){discriminatorArg}));");


Doing ??= new() shouldn't be necessary for every entry. Prefer something along the lines of:

PolymorphismOptions options = new(); options.DerivedTypes.Add(new JsonDerivedType(...)); options.DerivedTypes.Add(new JsonDerivedType(...)); options.DerivedTypes.Add(new JsonDerivedType(...)); jsonTypeInfo.PolymorphismOptions = options;

Fixed in 4712c26. The emitter now creates a single polymorphismOptions variable, adds all derived types, then assigns to jsonTypeInfo.PolymorphismOptions.

eiriktsarpalis · 2026-04-23T18:34:53Z

                    ClassType = classType,
                    PrimitiveTypeKind = primitiveTypeKind,
                    IsPolymorphic = isPolymorphic,
+                    OpenGenericDerivedTypes = openGenericDerivedTypes?.ToImmutableEquatableArray(),


@copilot ensure the derived types are sorted by some key for deterministic compilation.

Added in 4712c26. Derived types are sorted by DerivedType.FullyQualifiedName via OrderBy before creating the immutable array.

eiriktsarpalis · 2026-04-23T18:35:45Z

                        var derivedType = (ITypeSymbol)attributeData.ConstructorArguments[0].Value!;
-                        EnqueueType(derivedType, typeToGenerate.Mode);
+
+                        // Open generic derived types (e.g. typeof(Derived<>)) are resolved at compile time


Move logic to a helper method that optionally returns a closed type to enqueue, or null to emit a warning diagnostic.

Refactored in 4712c26. TryResolveOpenGenericDerivedType now accepts ITypeSymbol baseType and does the INamedTypeSymbol { IsGenericType: true } check internally, returning false for non-generic base types. The caller is simplified to TryResolveOpenGenericDerivedType(unboundDerived, typeToGenerate.Type, out ...).

eiriktsarpalis · 2026-04-23T18:36:12Z

+        /// for the reflection-based resolver path (not source generator / AOT).
+        /// </summary>
+        [RequiresUnreferencedCode(JsonSerializer.SerializationUnreferencedCodeMessage)]
+        [RequiresDynamicCode(JsonSerializer.SerializationRequiresDynamicCodeMessage)]


Please remove.

Removed in 4712c26.

…actor parser, remove doc comment, fix null guard, conditional PopulatePolymorphismMetadata Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/e620b332-1edf-45ad-9d85-bc907429e008 Co-authored-by: eiriktsarpalis <6668460+eiriktsarpalis@users.noreply.github.com>

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/e620b332-1edf-45ad-9d85-bc907429e008 Co-authored-by: eiriktsarpalis <6668460+eiriktsarpalis@users.noreply.github.com>

Copilot · 2026-04-23T19:19:16Z

@copilot please find the location that handles source generated JsonTypeInfo and currently resolved derived type metadata using attribute resolution. This is no longer necessary given that the source ...

Addressed in 4712c26. PopulatePolymorphismMetadata() now checks if (_polymorphismOptions is not null) return; — when the source generator has already set polymorphism options, attribute-based resolution is skipped. For binary compatibility with older source-generated code (which didn't set options), the attribute logic is preserved as the fallback.

Initial plan

fc2d297

Copilot AI assigned Copilot and eiriktsarpalis Apr 23, 2026

Copilot AI requested review from Copilot and removed request for Copilot April 23, 2026 11:43

Copilot started work on behalf of eiriktsarpalis April 23, 2026 11:43 View session

Copilot AI linked an issue Apr 23, 2026 that may be closed by this pull request

Support open generics in polymorphic serialization in System.Text.Json #127317

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github-actions Bot added the area-Serialization label Apr 23, 2026

Copilot AI requested review from Copilot and removed request for Copilot April 23, 2026 12:23

Add comprehensive tests for open generic polymorphic serialization

c9e89e7

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Copilot AI requested review from Copilot and removed request for Copilot April 23, 2026 12:26

Copilot AI requested review from Copilot and removed request for Copilot April 23, 2026 12:32

Copilot AI changed the title ~~[WIP] Support open generics in polymorphic serialization~~ Support open generics in polymorphic serialization in System.Text.Json Apr 23, 2026

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Copilot AI requested review from Copilot and removed request for Copilot April 23, 2026 19:16

Fix alphabetical ordering of type ref constants in Emitter

05f70b2

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/e620b332-1edf-45ad-9d85-bc907429e008 Co-authored-by: eiriktsarpalis <6668460+eiriktsarpalis@users.noreply.github.com>

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Architecture

Rejected patterns (throw InvalidOperationException)

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Rejected patterns (throw `InvalidOperationException`)