Support IReadOnlyDictionary<string, object/JsonElement> as JsonExtensionData

[Copilot]

• Explore repository structure and understand current JsonExtensionData implementation
• Build baseline to verify environment setup
• Update IsValidExtensionDataProperty to accept IReadOnlyDictionary types (with proper validation)
• Update CreateExtensionDataProperty logic to handle IReadOnlyDictionary instantiation (checking assignability)
• Update JsonExtensionDataAttribute documentation
• Add tests for IReadOnlyDictionary extension data support
• Fix validation to prevent ImmutableDictionary regression
• Add new test classes to source generation contexts
• Update source generator validation logic to support IReadOnlyDictionary
• Fix null reference warning in source generator type construction

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support using any dictionary as JsonExtensionData</issue_title>
<issue_description>IMHO, IReadOnlyDictionary should work identical to IDictionary when using with JsonExtensionData attribute.

[JsonExtensionData]
public IReadOnlyDictionary<string, JsonElement> Payload { get; set; }

Currently it fails with error:

System.InvalidOperationException: The data extension property 'Payload' does not match the required signature of IDictionary<string, JsonElement> or IDictionary<string, object>.

This is analogous to collection properties which can be defined as List<T>, IList<T> or IReadOnlyList<T> etc.</issue_description>

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

@safern > 'C:\Projects\dotnet-runtime\artifacts\bin\System.Text.Json.Tests\Debug\System.Text.Json.Tests.dll' is valid.

Anipik it seems like the path is missing the TFM, was this fixed with yesterday's PR as well?

I think if you don't open with build -vs ... you won't be able to run the tests because we're setting some environment variables needed for VSTest to run against the lively built shared framework. If it didn't open with preview, you might be missing the Preview devenv in the path, or you're opening a developer command prompt that is not preview, could you make sure that is not the case? </comment_new>
<comment_new>@krwq
@Romanx I assume you don't work on that anymore. This will be much easier to fix once populating objects is supported. There is a chance I will address this as part of that work but no promises (even if I don't it's going to be much easier to do).</comment_new>

Fixes #31645

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[stephentoub]

@copilot, there are test failures:

C:\h\w\A1FD0945\w\A96A09B8\e>"C:\h\w\A1FD0945\p\dotnet.exe" exec --runtimeconfig System.Text.Json.SourceGeneration.Roslyn4.4.Tests.runtimeconfig.json --depsfile System.Text.Json.SourceGeneration.Roslyn4.4.Tests.deps.json xunit.console.dll System.Text.Json.SourceGeneration.Roslyn4.4.Tests.dll -xml testResults.xml -nologo -nocolor -notrait category=IgnoreForCI -notrait category=OuterLoop -notrait category=failing  
  Discovering: System.Text.Json.SourceGeneration.Roslyn4.4.Tests (method display = ClassAndMethod, method display options = None)
  Discovered:  System.Text.Json.SourceGeneration.Roslyn4.4.Tests (found 4005 of 4061 test cases)
  Starting:    System.Text.Json.SourceGeneration.Roslyn4.4.Tests (parallel test collections = on [2 threads], stop on fail = off)
    System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Default.IReadOnlyDictionary_ExtensionPropertyIgnoredWhenWritingDefault [FAIL]
      System.NotSupportedException : JsonTypeInfo metadata for type 'System.Text.Json.Serialization.Tests.ExtensionDataTests+ClassWithIReadOnlyDictionaryExtensionPropertyAsObject' was not provided by TypeInfoResolver of type 'System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Default+ExtensionDataTestsContext_Default'. If using source generation, ensure that all root types passed to the serializer have been annotated with 'JsonSerializableAttribute', along with any types that might be serialized polymorphically.
      Stack Trace:
        /_/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs(855,0): at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType(Type type, IJsonTypeInfoResolver resolver)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(126,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(153,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type type, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(45,0): at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type inputType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(53,0): at System.Text.Json.JsonSerializer.GetTypeInfo[T](JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Write.String.cs(32,0): at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSerializerWrapper.SourceGen.cs(29,0): at System.Text.Json.SourceGeneration.Tests.StringSerializerWrapper.SerializeWrapper[T](T value, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs(1524,0): at System.Text.Json.Serialization.Tests.ExtensionDataTests.IReadOnlyDictionary_ExtensionPropertyIgnoredWhenWritingDefault()
        --- End of stack trace from previous location ---
    System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Default.IReadOnlyDictionary_ObjectExtensionPropertyRoundTrip [FAIL]
      System.NotSupportedException : JsonTypeInfo metadata for type 'System.Text.Json.Serialization.Tests.ExtensionDataTests+ClassWithIReadOnlyDictionaryExtensionPropertyAsObjectWithProperty' was not provided by TypeInfoResolver of type 'System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Default+ExtensionDataTestsContext_Default'. If using source generation, ensure that all root types passed to the serializer have been annotated with 'JsonSerializableAttribute', along with any types that might be serialized polymorphically.
      Stack Trace:
        /_/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs(855,0): at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType(Type type, IJsonTypeInfoResolver resolver)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(126,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(153,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type type, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(45,0): at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type inputType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(53,0): at System.Text.Json.JsonSerializer.GetTypeInfo[T](JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.String.cs(51,0): at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSerializerWrapper.SourceGen.cs(41,0): at System.Text.Json.SourceGeneration.Tests.StringSerializerWrapper.DeserializeWrapper[T](String json, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs(1491,0): at System.Text.Json.Serialization.Tests.ExtensionDataTests.IReadOnlyDictionary_ObjectExtensionPropertyRoundTrip()
        --- End of stack trace from previous location ---
    System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Default.IReadOnlyDictionary_JsonElementExtensionPropertyRoundTrip [FAIL]
      System.NotSupportedException : JsonTypeInfo metadata for type 'System.Text.Json.Serialization.Tests.ExtensionDataTests+ClassWithIReadOnlyDictionaryExtensionPropertyAsJsonElementWithProperty' was not provided by TypeInfoResolver of type 'System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Default+ExtensionDataTestsContext_Default'. If using source generation, ensure that all root types passed to the serializer have been annotated with 'JsonSerializableAttribute', along with any types that might be serialized polymorphically.
      Stack Trace:
        /_/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs(855,0): at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType(Type type, IJsonTypeInfoResolver resolver)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(126,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(153,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type type, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(45,0): at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type inputType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(53,0): at System.Text.Json.JsonSerializer.GetTypeInfo[T](JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.String.cs(51,0): at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSerializerWrapper.SourceGen.cs(41,0): at System.Text.Json.SourceGeneration.Tests.StringSerializerWrapper.DeserializeWrapper[T](String json, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs(1508,0): at System.Text.Json.Serialization.Tests.ExtensionDataTests.IReadOnlyDictionary_JsonElementExtensionPropertyRoundTrip()
        --- End of stack trace from previous location ---
    System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Metadata.IReadOnlyDictionary_ExtensionPropertyIgnoredWhenWritingDefault [FAIL]
      System.NotSupportedException : JsonTypeInfo metadata for type 'System.Text.Json.Serialization.Tests.ExtensionDataTests+ClassWithIReadOnlyDictionaryExtensionPropertyAsObject' was not provided by TypeInfoResolver of type 'System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Metadata+ExtensionDataTestsContext_Metadata'. If using source generation, ensure that all root types passed to the serializer have been annotated with 'JsonSerializableAttribute', along with any types that might be serialized polymorphically.
      Stack Trace:
        /_/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs(855,0): at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType(Type type, IJsonTypeInfoResolver resolver)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(126,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(153,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type type, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(45,0): at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type inputType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(53,0): at System.Text.Json.JsonSerializer.GetTypeInfo[T](JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Write.String.cs(32,0): at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSerializerWrapper.SourceGen.cs(29,0): at System.Text.Json.SourceGeneration.Tests.StringSerializerWrapper.SerializeWrapper[T](T value, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs(1524,0): at System.Text.Json.Serialization.Tests.ExtensionDataTests.IReadOnlyDictionary_ExtensionPropertyIgnoredWhenWritingDefault()
        --- End of stack trace from previous location ---
    System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Metadata.IReadOnlyDictionary_ObjectExtensionPropertyRoundTrip [FAIL]
      System.NotSupportedException : JsonTypeInfo metadata for type 'System.Text.Json.Serialization.Tests.ExtensionDataTests+ClassWithIReadOnlyDictionaryExtensionPropertyAsObjectWithProperty' was not provided by TypeInfoResolver of type 'System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Metadata+ExtensionDataTestsContext_Metadata'. If using source generation, ensure that all root types passed to the serializer have been annotated with 'JsonSerializableAttribute', along with any types that might be serialized polymorphically.
      Stack Trace:
        /_/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs(855,0): at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType(Type type, IJsonTypeInfoResolver resolver)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(126,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(153,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type type, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(45,0): at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type inputType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(53,0): at System.Text.Json.JsonSerializer.GetTypeInfo[T](JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.String.cs(51,0): at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSerializerWrapper.SourceGen.cs(41,0): at System.Text.Json.SourceGeneration.Tests.StringSerializerWrapper.DeserializeWrapper[T](String json, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs(1491,0): at System.Text.Json.Serialization.Tests.ExtensionDataTests.IReadOnlyDictionary_ObjectExtensionPropertyRoundTrip()
        --- End of stack trace from previous location ---
    System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Metadata.IReadOnlyDictionary_JsonElementExtensionPropertyRoundTrip [FAIL]
      System.NotSupportedException : JsonTypeInfo metadata for type 'System.Text.Json.Serialization.Tests.ExtensionDataTests+ClassWithIReadOnlyDictionaryExtensionPropertyAsJsonElementWithProperty' was not provided by TypeInfoResolver of type 'System.Text.Json.SourceGeneration.Tests.ExtensionDataTests_Metadata+ExtensionDataTestsContext_Metadata'. If using source generation, ensure that all root types passed to the serializer have been annotated with 'JsonSerializableAttribute', along with any types that might be serialized polymorphically.
      Stack Trace:
        /_/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs(855,0): at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType(Type type, IJsonTypeInfoResolver resolver)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(126,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs(153,0): at System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type type, Boolean fallBackToNearestAncestorType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(45,0): at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type inputType)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs(53,0): at System.Text.Json.JsonSerializer.GetTypeInfo[T](JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.String.cs(51,0): at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSerializerWrapper.SourceGen.cs(41,0): at System.Text.Json.SourceGeneration.Tests.StringSerializerWrapper.DeserializeWrapper[T](String json, JsonSerializerOptions options)
        /_/src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs(1508,0): at System.Text.Json.Serialization.Tests.ExtensionDataTests.IReadOnlyDictionary_JsonElementExtensionPropertyRoundTrip()
        --- End of stack trace from previous location ---
  Finished:    System.Text.Json.SourceGeneration.Roslyn4.4.Tests
=== TEST EXECUTION SUMMARY ===
   System.Text.Json.SourceGeneration.Roslyn4.4.Tests  Total: 7596, Errors: 0, Failed: 6, Skipped: 0, Time: 34.037s

[Copilot]

Pull Request Overview

This PR extends JsonExtensionData support to include IReadOnlyDictionary<string, object> and IReadOnlyDictionary<string, JsonElement> as valid extension data property types, alongside the existing IDictionary support.

• Adds validation logic to accept IReadOnlyDictionary interface types when Dictionary can be assigned to them
• Updates instantiation logic to create Dictionary instances for IReadOnlyDictionary extension data properties
• Enhanced documentation to reflect the new supported types

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
JsonTypeInfo.cs	Added validation logic for IReadOnlyDictionary extension data properties
JsonSerializer.Read.HandlePropertyName.cs	Added instantiation logic to create Dictionary instances for IReadOnlyDictionary properties
JsonExtensionDataAttribute.cs	Updated XML documentation to include IReadOnlyDictionary support
ExtensionDataTests.cs	Added comprehensive tests for IReadOnlyDictionary extension data functionality
DefaultJsonTypeInfoResolverTests.JsonPropertyInfo.cs	Added test cases for IReadOnlyDictionary types in extension data validation

[Copilot]

@copilot, there are test failures:

C:\h\w\A1FD0945\w\A96A09B8\e>"C:\h\w\A1FD0945\p\dotnet.exe" exec --runtimeconfig System.Text.Json.SourceGeneration.Roslyn4.4.Tests.runtimeconfig.json --depsfil...

Fixed in commit 3f9c7dc. The source generator's IsValidDataExtensionPropertyType method has been updated to also accept IReadOnlyDictionary types (with the same assignability check to exclude ImmutableDictionary), and the new test classes have been registered in both source generation contexts.

[stephentoub]

@copilot, please ensure you build and fully test all changes. There are build failures:

Build FAILED.

D:\a\_work\1\s\src\libraries\System.Text.Json\gen\Helpers\KnownTypeSymbols.cs(201,105): error CS8604: Possible null reference argument for parameter 'typeArguments' in 'INamedTypeSymbol INamedTypeSymbol.Construct(params ITypeSymbol[] typeArguments)'. [D:\a\_work\1\s\src\libraries\System.Text.Json\gen\System.Text.Json.SourceGeneration.Roslyn4.4.csproj::TargetFramework=net10.0]
    0 Warning(s)
    1 Error(s)

[Copilot]

@copilot, please ensure you build and fully test all changes. There are build failures:

Build FAILED.

...

Fixed in commit 8f4a3e0. The StringJsonElementDictionaryType property now properly checks for null before constructing the type to avoid the CS8604 warning. All source generation tests now pass.