Support type-level JsonIgnoreCondition via [JsonIgnore] on classes, structs, and interfaces

[Copilot]

Description

[JsonIgnore(Condition = ...)] can now be applied at the type level to set a default JsonIgnoreCondition for all properties and fields, eliminating per-property duplication:

[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
class MyPoco
{
    public string? PropA { get; set; }
    public string? PropB { get; set; }
    public string? PropC { get; set; }
}

Property-level [JsonIgnore] overrides the type-level setting. The type-level annotation also overrides the global JsonSerializerOptions.DefaultIgnoreCondition — for example, a type-level WhenWritingNull takes precedence over a global WhenWritingDefault. For non-nullable value types where WhenWritingNull is inapplicable, the ignore condition is set to Never to ensure the type-level annotation still overrides the global JSO policy. JsonIgnoreCondition.Always is rejected at the type level as invalid configuration — the reflection path throws InvalidOperationException, and the source generator emits SYSLIB1226 warning diagnostic and ignores the attribute.

Changes

• API surface: Expanded JsonIgnoreAttribute AttributeUsage to include Class | Struct | Interface, with API compat suppressions for the AttributeUsage change in both the shared framework baseline (ApiCompatBaseline.NetCoreAppLatestStable.xml) and the package-level baseline (CompatibilitySuppressions.xml for net10.0, net462, netstandard2.0)
• Reflection resolver (DefaultJsonTypeInfoResolver.Helpers.cs): Resolves type-level [JsonIgnore] once per type in PopulateProperties and passes the resolved JsonIgnoreCondition? parametrically through AddMembersDeclaredBySuperType → AddMember → CreatePropertyInfo, avoiding redundant per-member attribute lookups. Uses typeInfo.Type (root serialized type) so type-level attributes on derived classes apply to inherited base-class properties. Throws InvalidOperationException for JsonIgnoreCondition.Always at the type level. For WhenWritingNull on non-nullable value types, sets the ignore condition to Never to prevent the global DefaultIgnoreCondition from taking over.
• Source generator (JsonSourceGenerator.Parser.cs): Reads type-level [JsonIgnore] in ProcessTypeCustomAttributes using cached JsonIgnoreAttributeType symbol from KnownTypeSymbols.cs, threads it through to property parsing as a fallback. Emits SYSLIB1226 diagnostic for JsonIgnoreCondition.Always at the type level. Applies the same WhenWritingNull → Never mapping for non-nullable value types.
• Diagnostic: Added SYSLIB1226 diagnostic descriptor, resource strings (including XLF translations), and updated list-of-diagnostics.md
• Tests: Covers WhenWritingNull, WhenWritingDefault, Always (invalid config), property-level override, struct, inheritance, deserialization, and type-level annotation overriding global JsonSerializerOptions.DefaultIgnoreCondition scenarios across both reflection and source gen paths. Includes source generator unit test for the SYSLIB1226 diagnostic.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>It should be possible to specify JsonIgnoreCondition's on the type level</issue_title>
<issue_description>## Motivation

There's a substantial amount of duplication in the following code:

class MyPoco
{
    [JsonIgnore(JsonIgnoreCondition.WhenWritingNull)]
    public string? PropA { get; set; }

    [JsonIgnore(JsonIgnoreCondition.WhenWritingNull)]
    public string? PropB { get; set; }

    [JsonIgnore(JsonIgnoreCondition.WhenWritingNull)]
    public string? PropC { get; set; }
}

that could be eliminated using a type-level declaration:

[JsonIgnore(JsonIgnoreCondition.WhenWritingNull)]
class MyPoco
{
    public string? PropA { get; set; }

    public string? PropB { get; set; }

    public string? PropC { get; set; }
}

API Proposal

namespace System.Text.Json.Serialization;

-[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface | AttributeTargets.AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
public partial class JsonIgnoreAttribute;

cc @stephentoub</issue_description>

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

• Fixes It should be possible to specify JsonIgnoreCondition's on the type level #108231

---

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

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json
See info in area-owners.md if you want to be subscribed.

[eiriktsarpalis]

@copilot

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/graphql
  • Triggering command: /usr/bin/gh gh pr checks 114212 --json name,state (http block)
  • Triggering command: /usr/bin/gh /usr/bin/gh pr checks 114212 --repo dotnet/runtime (http block)
  • Triggering command: /usr/bin/gh gh pr view --json number,url,headRefName (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[eiriktsarpalis]

@copilot Please rebase changes and fix merge conflicts.

[eiriktsarpalis]

@copilot Please rebase changes and fix merge conflicts.

[Copilot]

@copilot Please rebase changes and fix merge conflicts.

Merged latest main and resolved the conflict in JsonSourceGeneratorDiagnosticsTests.cs (kept both our JsonIgnoreConditionAlwaysOnTypeWarns test and main's new Diagnostic_HasPragmaSuppressibleLocation test). Commit 8af167a.

[eiriktsarpalis]

@copilot add testing showing that the attribute annotation overrides the global JSO setting.

[Copilot]

Pull request overview

Adds support for applying [JsonIgnore(Condition = ...)] at the type level to provide a default JsonIgnoreCondition for all members, with consistent behavior across reflection-based metadata and the System.Text.Json source generator.

Changes:

• Expanded JsonIgnoreAttribute to allow usage on class, struct, and interface, with API compat/package validation suppressions.
• Implemented type-level JsonIgnoreCondition fallback in both reflection resolver and source generator, including invalid-configuration handling for Always.
• Added new source-generator warning SYSLIB1226 and comprehensive tests for the new behavior.

Reviewed changes

Copilot reviewed 28 out of 28 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/apicompat/ApiCompatBaseline.NetCoreAppLatestStable.xml	Suppresses API compat diagnostic for the AttributeUsage change.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Attributes/JsonIgnoreAttribute.cs	Extends attribute targets and updates XML docs to describe type-level behavior.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/DefaultJsonTypeInfoResolver.Helpers.cs	Adds reflection-path fallback to type-level ignore condition and rejects Always.
src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs	Adds generator support for type-level ignore condition fallback + emits SYSLIB1226 for invalid Always.
src/libraries/System.Text.Json/gen/JsonSourceGenerator.DiagnosticDescriptors.cs	Introduces diagnostic descriptor for SYSLIB1226.
src/libraries/System.Text.Json/gen/Helpers/KnownTypeSymbols.cs	Adds cached symbol lookup for JsonIgnoreAttribute.
src/libraries/System.Text.Json/gen/Resources/Strings.resx	Adds resource strings for SYSLIB1226 title/message.
src/libraries/System.Text.Json/gen/Resources/xlf/Strings.*.xlf	Adds localization entries for the new diagnostic strings.
docs/project/list-of-diagnostics.md	Documents SYSLIB1226 meaning in the diagnostics list.
src/libraries/System.Text.Json/ref/System.Text.Json.cs	Updates reference assembly metadata for new attribute usage targets.
src/libraries/System.Text.Json/src/CompatibilitySuppressions.xml	Adds package validation baseline suppressions for the AttributeUsage change across target TFMs.
src/libraries/System.Text.Json/tests/Common/PropertyVisibilityTests.cs	Adds reflection-path tests for type-level ignore semantics, overrides, structs, and inheritance.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/PropertyVisibilityTests.cs	Adds source-gen-path test ensuring type-level Always is ignored (serialized normally).
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/System.Text.Json.SourceGeneration.Tests.targets	Suppresses SYSLIB1226 in source-gen integration tests (covered explicitly elsewhere).
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/CompilationHelper.cs	Adds compilation helper for SYSLIB1226 unit-test scenario.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorDiagnosticsTests.cs	Adds unit test validating SYSLIB1226 warning and location.

---

You can also share your feedback on Copilot code review. Take the survey.

[Copilot]

@copilot add testing showing that the attribute annotation overrides the global JSO setting.

Added JsonIgnoreCondition_TypeLevel_OverridesGlobalJSO test that verifies the type-level [JsonIgnore(Condition = WhenWritingNull)] overrides the global JsonSerializerOptions.DefaultIgnoreCondition = WhenWritingDefault. Also fixed a bug in both the reflection resolver and source generator where WhenWritingNull on non-nullable value types was leaving ignoreCondition as null, allowing the global JSO policy to take over. Now it's set to Never in that case, ensuring the type-level annotation always takes precedence. Commit a886e49.

[tarekgh]

LGTM. I added one comment on issue originally raised by copilot if you want to consider it.

[eiriktsarpalis]

/ba-g unrelated mono build failures.