Fix duplicate serialization of virtual properties with JsonPropertyName on base class

[Copilot]

• Understand the issue: virtual property with JsonPropertyName on base class is serialized twice when overridden in derived class
• Modify PropertyHierarchyResolutionState to track properties by CLR member name for virtual property override detection
• Modify AddPropertyWithConflictResolution to skip base class virtual properties that have been overridden
• Apply similar fix to source generator in JsonSourceGenerator.Parser.cs
• Fix CreatePropertyInfoCore in JsonMetadataServices.Helpers.cs to set IsVirtual property
• Add tests to validate the fix
• Add test types to source generation context
• Build and run tests
• Run code review and codeql check
• Address PR feedback: removed regression test comment

Original prompt

---

This section details on the original issue you should resolve

<issue_title>System.text.json serializes virtual property decorated with JsonPropertyName attribute twice.</issue_title>
<issue_description>### Description

System.text.json serializes property decorated with JsonPropertyName attribute in the base class is recording 2 records in the output json , one with property name as key and a second with name specified in the attribute.

Reproduction Steps

Console.WriteLine(JsonSerializer.Serialize(new Derived()));

public class Base
{
    [JsonPropertyName("test")]
    public virtual string Id { get => "base"; set { } }
}

public class Derived : Base
{
    public override string Id { get => "derived"; set { } }
}

Expected behavior

only single entry with name specified in the JsonPropertyName should be adde to json.

Actual behavior

{"Id":"derived","test":"derived"} -> added twice with different names.

Regression?

We have seen this since the initial version.

Known Workarounds

I see two possible workarounds:

1. Try using the base type as the generic parameter when creating your container object. Unlike Json.NET, STJ is sensitive when it comes to what type is being declared statically at the serialization callsite, or
2. Replicate the JsonPropertyName property in the overrides. It's more boilerplate, but at least it's the best way to get the desired behavior while we ship a fix.

Configuration

.NET 7

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

System.text.json serializes property decorated with JsonPropertyName attribute in the base class is recording 2 records in the output json , one with property name as key and a second with name specified in the attribute.

Reproduction Steps

Console.WriteLine(JsonSerializer.Serialize(new Derived()));

public class Base
{
[JsonPropertyName("test")]
public virtual string Id { get => "base"; set { } }
}

public class Derived : Base
{
public override string Id { get => "derived"; set { } }
}

Expected behavior

only single entry with name specified in the JsonPropertyName should be adde to json.

Actual behavior

{"Id":"derived","test":"derived"} -> added twice with different names.

Regression?

We have seen this since the initial version.

Known Workarounds

I see two possible workarounds:

1. Try using the base type as the generic parameter when creating your container object. Unlike Json.NET, STJ is sensitive when it comes to what type is being declared statically at the serialization callsite, or
2. Replicate the JsonPropertyName property in the overrides. It's more boilerplate, but at least it's the best way to get the desired behavior while we ship a fix.

Configuration

.NET 7

Other information

No response

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

@eiriktsarpalis I can reproduce in .NET 8. This falls under the same umbrella of issues described in https://github.com//issues/73255, namely it concerns attribute resolution of virtual properties. We should try to address all these issues in unison, although it would admit the probability of breaking changes.

In the meantime, you could try the following workarounds:

1. Serialize the value using the base type as the generic parameter: JsonSerializer.Serialize<Base>(new Derived()); or
2. Apply the same JsonPropertyName attribute to the overridden property.</comment_new>

• Fixes System.text.json serializes virtual property decorated with JsonPropertyName attribute twice. #92780

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Pull request overview

This PR fixes a bug where virtual properties with [JsonPropertyName] on a base class were serialized twice when overridden in a derived class without the attribute. The root cause was that property conflict resolution only tracked properties by JSON name, so when the derived override lacked the attribute, the different JSON names ("Id" vs "test") prevented collision detection.

Key Changes

• Added OverriddenVirtualProperties dictionary to track virtual properties by CLR member name
• Added early-exit check in AddPropertyWithConflictResolution to detect virtual property overrides even when JSON names differ
• Applied the same fix to both runtime (JsonTypeInfo.cs) and source generator (JsonSourceGenerator.Parser.cs)
• Set IsVirtual property in CreatePropertyInfoCore for source-generated metadata
• Added comprehensive tests covering the bug scenario and related edge cases

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfo.cs	Added OverriddenVirtualProperties dictionary to PropertyHierarchyResolutionState and implemented virtual property override detection logic in AddPropertyWithConflictResolution
src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs	Mirrored the runtime changes in the source generator: added OverriddenVirtualMembers dictionary and override detection logic
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.Helpers.cs	Set IsVirtual property in CreatePropertyInfoCore to ensure source-generated metadata has this flag set correctly
src/libraries/System.Text.Json/tests/Common/PropertyNameTests.cs	Added three test methods covering virtual property scenarios and test class definitions
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/PropertyNameTests.cs	Registered new test classes in both Metadata and Default source generation contexts

[Copilot]

The comment references issue #96998, but the PR description indicates this fixes issue #92780. Please update the issue number in the comment to match the correct issue being fixed.

[stephentoub]

@copilot, you can remove the comment about this being a regression test.

[Copilot]

Removed in 7954786.

[stephentoub]

@copilot, you can remove the comment about this being a regression test.

[stephentoub]

@copilot, was this a bug on its own? Are there other issues that stemmed from this not being copied, or was it otherwise benign? Are there other properties that should be copied but aren't?

[Copilot]

Yes, this was a latent bug - IsVirtual was populated by the source generator into JsonPropertyInfoValues<T>.IsVirtual but never copied to JsonPropertyInfo.IsVirtual. This was benign before because IsVirtual on JsonPropertyInfo was only checked when there was a JSON name collision (handled by the existing conflict resolution logic). My fix introduced a new check that uses IsVirtual even when JSON names differ, which exposed the missing copy.

I verified all other properties in JsonPropertyInfoValues<T> are being copied: IsProperty, IsPublic, DeclaringType, PropertyTypeInfo, Converter, Getter, Setter, IgnoreCondition, HasJsonInclude, IsExtensionData, NumberHandling, PropertyName, JsonPropertyName, and AttributeProviderFactory are all handled.