Fix JsonSerializer.Serialize producing invalid JSON for [JsonExtensionData] with JsonObject #122838

Copilot · 2026-01-03T21:08:44Z

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.

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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<string, JsonElement>' or 'IDictionary<string, object>', 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

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

…sonObject When serializing an object containing a property marked with [JsonExtensionData] attribute of type JsonObject, the serializer was producing invalid JSON like {"Id":1,{"nested":true}} instead of the correct {"Id":1,"nested":true}. The fix modifies TryWriteDataExtensionProperty in JsonConverterOfT.cs to write the JsonObject extension data contents without the wrapping braces, similar to how dictionary extension data is handled. Also added tests to verify the fix for JsonObject extension data serialization. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs

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

Copilot · 2026-01-04T00:40:39Z

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs

+                Debug.Assert(Type == typeof(JsonObject));
+
+                // Write the JsonObject extension data contents without the wrapping braces.
+                // This is necessary because extension data properties should be flattened
+                // into the parent object, not nested as a separate object.
+                JsonObject jsonObject = (JsonObject)(object)value!;
+                foreach (KeyValuePair<string, JsonNode?> entry in jsonObject)
+                {
+                    writer.WritePropertyName(entry.Key);
+
+                    if (entry.Value is null)
+                    {
+                        writer.WriteNullValue();
+                    }
+                    else
+                    {
+                        entry.Value.WriteTo(writer, options);
+                    }
+                }
+
+                return true;


The depth validation is missing for the JsonObject extension data path. While WriteTo on JsonNode will eventually check depth through the Utf8JsonWriter, it's inconsistent with how the dictionary converter path handles depth checking (lines 486-488). Consider adding an explicit depth check before iterating, similar to the dictionary path, to fail fast and provide consistent error handling.

Copilot · 2026-01-04T00:40:39Z

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs

+                Debug.Assert(Type == typeof(JsonObject));
+
+                // Write the JsonObject extension data contents without the wrapping braces.
+                // This is necessary because extension data properties should be flattened
+                // into the parent object, not nested as a separate object.
+                JsonObject jsonObject = (JsonObject)(object)value!;
+                foreach (KeyValuePair<string, JsonNode?> entry in jsonObject)
+                {
+                    writer.WritePropertyName(entry.Key);
+
+                    if (entry.Value is null)
+                    {
+                        writer.WriteNullValue();
+                    }
+                    else
+                    {
+                        entry.Value.WriteTo(writer, options);
+                    }
+                }
+
+                return true;


The JsonObject extension data path doesn't maintain the WriteStack state by calling state.Push() and state.Pop() like the dictionary path does (lines 494, 512). This inconsistency could lead to issues with state tracking, particularly for continuation scenarios or complex nested structures. Consider adding proper state management to match the dictionary path's behavior.

PranavSenthilnathan · 2026-01-05T20:37:15Z

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs

            if (dictionaryConverter == null)
            {
                // If not JsonDictionaryConverter<T> then we are JsonObject.
                // Avoid a type reference to JsonObject and its converter to support trimming.


// Avoid a type reference to JsonObject and its converter to support trimming.

Is this not true anymore?

Removed that comment since we now use a direct type reference to JsonObject and call the new WriteContentsTo method. The previous comment was outdated - we already had a Debug.Assert(Type == typeof(JsonObject)) so there was already a type reference.

PranavSenthilnathan · 2026-01-05T20:44:07Z

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs

+                // This is necessary because extension data properties should be flattened
+                // into the parent object, not nested as a separate object.
+                JsonObject jsonObject = (JsonObject)(object)value!;
+                foreach (KeyValuePair<string, JsonNode?> entry in jsonObject)


This looks like it's copied from

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.cs

Lines 176 to 188 in 11e43c9

foreach (KeyValuePair<string, JsonNode?> entry in Dictionary)

{

writer.WritePropertyName(entry.Key);

if (entry.Value is null)

{

writer.WriteNullValue();

}

else

{

entry.Value.WriteTo(writer, options);

}

}

but one optimization that's wasn't copied was the special handling of a JsonElement backed JsonObject. This might be fine, but I think it's probably better to have this code in JsonObject and avoid duplicating it if possible.

Added JsonObject.WriteContentsTo() internal method in 7f5a477. It handles both the JsonElement-backed case (using EnumerateObject()) and the dictionary-backed case, avoiding code duplication with WriteTo().

stephentoub · 2026-01-05T22:20:15Z

@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 ---

…WriteContentsTo - Add internal WriteContentsTo method to JsonObject that writes properties without wrapping braces, handling both dictionary-backed and JsonElement-backed cases - Update TryWriteDataExtensionProperty to call the new method, avoiding code duplication - Register ClassWithJsonObjectExtensionDataAndProperty in source generator contexts to fix failing source gen tests - Remove unnecessary using directive Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Copilot · 2026-01-05T22:58:11Z

@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.

Initial plan

de1c56e

Copilot AI assigned Copilot and stephentoub Jan 3, 2026

Copilot started work on behalf of stephentoub January 3, 2026 21:09 View session

Copilot AI changed the title ~~[WIP] Fix JsonSerializer.Serialize for JsonExtensionData property~~ Fix JsonSerializer.Serialize producing invalid JSON for [JsonExtensionData] with JsonObject Jan 3, 2026

Copilot AI requested a review from stephentoub January 3, 2026 21:48

Copilot finished work on behalf of stephentoub January 3, 2026 21:48

stephentoub reviewed Jan 4, 2026

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Update src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs

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Update src/libraries/System.Text.Json/tests/Common/ExtensionDataTests.cs

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stephentoub marked this pull request as ready for review January 4, 2026 00:30

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Fix JsonSerializer.Serialize producing invalid JSON for [JsonExtensionData] with JsonObject #122838

Fix JsonSerializer.Serialize producing invalid JSON for [JsonExtensionData] with JsonObject #122838

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

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

	foreach (KeyValuePair<string, JsonNode?> entry in Dictionary)
	{
	writer.WritePropertyName(entry.Key);

	if (entry.Value is null)
	{
	writer.WriteNullValue();
	}
	else
	{
	entry.Value.WriteTo(writer, options);
	}
	}

Fix JsonSerializer.Serialize producing invalid JSON for [JsonExtensionData] with JsonObject #122838

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Fix JsonSerializer.Serialize producing invalid JSON for [JsonExtensionData] with JsonObject #122838

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Description

Customer Impact

Regression

Testing

Risk

Package authoring no longer needed in .NET 9

Reproduction Steps

Expected behavior

Actual behavior

Regression?

Known Workarounds

Configuration

Other information

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