Fix JsonSerializer.Serialize producing invalid JSON for [JsonExtensionData] with JsonObject

[Copilot]

Description

JsonSerializer.Serialize was producing invalid JSON when serializing objects with [JsonExtensionData] properties of type JsonObject. The output was missing the property name for extension data entries:

var mix = new Mix { Id = 1, Extra = new JsonObject { ["nested"] = true } };
JsonSerializer.Serialize(mix);
// Before: {"Id":1,{"nested":true}}  ← invalid JSON
// After:  {"Id":1,"nested":true}    ← correct

Root cause: TryWriteDataExtensionProperty in JsonConverterOfT.cs called TryWrite for JsonObject, which writes the full object including braces. Extension data should write only the contents (key-value pairs) without wrapping braces.

Fix: Added a new internal WriteContentsTo method to JsonObject that writes properties without wrapping braces, handling both dictionary-backed and JsonElement-backed cases with proper optimization. Updated TryWriteDataExtensionProperty to call this method, avoiding code duplication and maintaining consistency with how dictionary extension data is handled.

Customer Impact

Serialization of any class with [JsonExtensionData] of type JsonObject produces malformed JSON that cannot be parsed.

Regression

No. Reproducible from .NET 6 through .NET 8+. Deserialization works correctly; only serialization was affected.

Testing

• Added 4 targeted tests covering: valid JSON output, round-trip, empty JsonObject, and null JsonObject
• All 49,808 existing System.Text.Json.Tests pass
• All 7,626 source generation tests pass
• Registered new test class in source generator contexts (ExtensionDataTestsContext_Metadata and ExtensionDataTestsContext_Default)

Risk

Low. Change is minimal and isolated to the JsonObject branch of extension data serialization. Dictionary extension data path is unchanged.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>JsonSerializer.Serialize produces invalid JSON for [JsonExtensionData] property</issue_title>
<issue_description>### Description

When serializing object containing property marked with [JsonExtensionData] attribute, serializer produces invalid JSON like this one: {"Id":1,{"nested":true}}

Reproduction Steps

Run the following code

var mix = new Mix
{
    Id = 1,
    Extra = new() { ["nested"] = true, }
};

var text = System.Text.Json.JsonSerializer.Serialize(mix);
Console.WriteLine(text);
// output {"Id":1,{"nested":true}}


public class Mix
{
    public int Id { get; set; }

    [System.Text.Json.Serialization.JsonExtensionData]
    public System.Text.Json.Nodes.JsonObject? Extra { get; set; }
}

Expected behavior

Correct JSON like {"Id":1,"nested":true} or at least valid JSON as if there was not [JsonExtensionData] attribute ({"Id":1,"Extra":{"nested":true}} ).

Actual behavior

Invalid JSON {"Id":1,{"nested":true}}

Regression?

Reproducible at least on .Net 6 to .Net 8

Known Workarounds

No response

Configuration

.Net 8
Windows 11
x64

Other information

No response</issue_description>

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

@ Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis See info in [area-owners.md](https://github.com/dotnet/runtime/blob/main/docs/area-owners.md) if you want to be subscribed.

Issue Details

---

Description

When serializing object containing property marked with [JsonExtensionData] attribute, serializer produces invalid JSON like this one: {"Id":1,{"nested":true}}

Reproduction Steps

Run the following code

var mix = new Mix
{
    Id = 1,
    Extra = new() { ["nested"] = true, }
};

var text = System.Text.Json.JsonSerializer.Serialize(mix);
Console.WriteLine(text);
// output {"Id":1,{"nested":true}}


public class Mix
{
    public int Id { get; set; }

    [System.Text.Json.Serialization.JsonExtensionData]
    public System.Text.Json.Nodes.JsonObject? Extra { get; set; }
}

Expected behavior

Correct JSON like {"Id":1,"nested":true} or at least valid JSON as if there was not [JsonExtensionData] attribute ({"Id":1,"Extra":{"nested":true}} ).

Actual behavior

Invalid JSON {"Id":1,{"nested":true}}

Regression?

Reproducible at least on .Net 6 to .Net 8

Known Workarounds

No response

Configuration

.Net 8
Windows 11
x64

Other information

No response

Author:	KalininAndreyVictorovich
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

@eiriktsarpalis Can confirm that this occurs. It seems we never added testing for the serialization scenario -- deserialization appears to be working as expected. @eiriktsarpalis The documentation is correctly stating that `JsonObject` is one of the supported types. However there is a bug specifically impacting serialization for the particular type (FWIW `JsonExtensionData` is a feature primarily oriented towards dserialization). @eiriktsarpalis You're right, I misread the documentation which appears to be out of date. The correct statement on supported types can actually be found in the error messages of the implementation itself:

runtime/src/libraries/System.Text.Json/gen/Resources/Strings.resx

Lines 150 to 152 in 2d751ca

	<data name="DataExtensionPropertyInvalidFormat" xml:space="preserve">
	<value>The data extension property '{0}.{1}' is invalid. It must implement 'IDictionary&lt;string, JsonElement&gt;' or 'IDictionary&lt;string, object&gt;', or be 'JsonObject'.</value>
	</data>

In other words, what I mentioned earlier holds. JsonObject is supported and there is a bug specifically concerning serialization.</comment_new>
<comment_new>@eiriktsarpalis
It would help if you could file a separate issue in dotnet-api-docs. Thanks!</comment_new>

• Fixes JsonSerializer.Serialize produces invalid JSON for [JsonExtensionData] property #97225

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[Copilot]

Pull request overview

This PR fixes a bug where JsonSerializer.Serialize produced invalid JSON when serializing objects with [JsonExtensionData] properties of type JsonObject. The output was missing property names and directly including object braces (e.g., {"Id":1,{"nested":true}}), which is malformed JSON.

Key Changes:

• Modified JsonConverterOfT.cs to iterate over JsonObject entries and write each key-value pair directly, ensuring extension data is properly flattened into the parent object
• Added comprehensive test coverage for JsonObject extension data serialization scenarios

Reviewed changes

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

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs	Replaced the TryWrite call for JsonObject extension data with explicit iteration and property-by-property writing to flatten extension data into the parent object
src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs	Added 4 new tests covering valid JSON output, round-trip serialization, empty JsonObject, and null JsonObject scenarios, plus a test class definition

[stephentoub]

@copilot, please address the feedback. Also please address the failing tests, e.g.

System.NotSupportedException : JsonTypeInfo metadata for type 'System.Text.Json.Serialization.Tests.ExtensionDataTests+ClassWithJsonObjectExtensionDataAndProperty' 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
   at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType(Type type, IJsonTypeInfoResolver resolver) in /_/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs:line 855
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType) in /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs:line 126
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type type, Boolean fallBackToNearestAncestorType) in /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs:line 153
   at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type inputType) in /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs:line 45
   at System.Text.Json.JsonSerializer.GetTypeInfo[T](JsonSerializerOptions options) in /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs:line 53
   at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options) in /_/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Write.String.cs:line 32
   at System.Text.Json.SourceGeneration.Tests.StringSerializerWrapper.SerializeWrapper[T](T value, JsonSerializerOptions options) in /_/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSerializerWrapper.SourceGen.cs:line 29
   at System.Text.Json.Serialization.Tests.ExtensionDataTests.SerializeJsonObjectExtensionData_EmptyJsonObject() in /_/src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs:line 887
--- End of stack trace from previous location ---

[Copilot]

@copilot, please address the feedback. Also please address the failing tests, e.g.

System.NotSupportedException : JsonTypeInfo metadata for type 'System.Text.Json.Serialization.Tests.ExtensionDa...

Fixed in 7f5a477. Added ClassWithJsonObjectExtensionDataAndProperty to both source generator contexts (ExtensionDataTestsContext_Metadata and ExtensionDataTestsContext_Default) and addressed the feedback by moving the extension data writing logic to JsonObject.WriteContentsTo() method. All tests now pass.