---

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>