Add IReadOnlySet support to System.Text.Json #120306

sander1095 · 2025-10-01T20:35:27Z

This pull request adds support for serializing and deserializing IReadOnlySet<T> collections in System.Text.Json for .NET versions greater or equal to 5.0. The changes are carefully scoped to only apply when targeting compatible frameworks, and include updates to the source generator, converter infrastructure, and test coverage.

Support for IReadOnlySet collections:

Added a new converter, IReadOnlySetOfTConverter, to handle serialization and deserialization of IReadOnlySet<T> types, falling back to HashSet<T> for deserialization. (src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/IReadOnlySetOfTConverter.cs)
Updated the source generator and metadata services to recognize and emit code for IReadOnlySet<T>, including new APIs like CreateIReadOnlySetInfo and corresponding enum values and method mappings. (src/libraries/System.Text.Json/gen/Helpers/KnownTypeSymbols.cs, src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs, src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs, src/libraries/System.Text.Json/gen/Model/CollectionType.cs, src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.Collections.cs, src/libraries/System.Text.Json/ref/System.Text.Json.cs) [1] [2] [3] [4] [5] [6]
Integrated the new converter into the converter factory logic so that IReadOnlySet<T> types are automatically handled when encountered. (src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/IEnumerableConverterFactory.cs)
Updated the project file to ensure the new converter is only compiled for supported target frameworks. (src/libraries/System.Text.Json/src/System.Text.Json.csproj)

Test coverage for IReadOnlySet:

Added comprehensive tests to verify serialization and deserialization of IReadOnlySet<T> and related scenarios, including inheritance and custom interfaces. (src/libraries/System.Text.Json/tests/Common/CollectionTests/CollectionTests.Generic.Read.cs) [1] [2] [3] [4]

Minor fixes:

Corrected a typo in an exception helper method name from ReturnsJsonConverterFactorty to ReturnsJsonConverterFactory. (src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs, src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterFactory.cs) [1] [2]
Fixes System.Text.Json cannot deserialize to IReadOnlySet #91875

This is based on existing test cases for IReadOnlyList and ISet

I'm pretty sure this is not the right direction, but it helps me with being able to run tests against an initial working implementation I'll work on next. I'll discuss the current #IF !NETFRAMEWORK code setup with the team when the PR is ready for a first review

This seems useful for code that deserializes to a type that deserializes to a Set type that implements both interfaces.

Copilot

Pull Request Overview

This PR adds support for IReadOnlySet<T> serialization/deserialization to System.Text.Json. The implementation creates a new converter that uses HashSet<T> as the concrete type for IReadOnlySet<T> interface deserialization.

Adds IReadOnlySetOfTConverter<TCollection, TElement> converter that deserializes JSON arrays to HashSet<T> instances
Integrates the new converter into the converter factory with appropriate conditional compilation
Adds comprehensive test coverage for both generic and object-typed IReadOnlySet<T> scenarios

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
IReadOnlySetOfTConverter.cs	New converter implementation for `IReadOnlySet<T>` using `HashSet<T>` as backing type
IEnumerableConverterFactory.cs	Integrates `IReadOnlySet<T>` converter into the factory with conditional compilation
System.Text.Json.csproj	Adds the new converter file to the project
ThrowHelper.Serialization.cs	Fixes typo in method name from "Factorty" to "Factory"
JsonConverterFactory.cs	Updates method call to use corrected spelling
Array.ReadTests.cs	Adds test methods for `IReadOnlySet<T>` deserialization
ReferenceHandlerTests.cs	Adds `JsonSerializable` attributes for `IReadOnlySet<T>` test classes
TestData.cs	Includes `IReadOnlySet<T>` test cases in success test data
TestClasses.cs	Adds test class definitions and minor formatting change to dictionary initialization

...xt.Json/src/System/Text/Json/Serialization/Converters/Collection/IReadOnlySetOfTConverter.cs

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

...Json/src/System/Text/Json/Serialization/Converters/Collection/IEnumerableConverterFactory.cs

...xt.Json/src/System/Text/Json/Serialization/Converters/Collection/IReadOnlySetOfTConverter.cs

eiriktsarpalis

This would also require updates to the source generator so IReadOnlySet is recognized and relevant code gets generated.

sander1095 · 2025-10-02T08:04:32Z

This would also require updates to the source generator so IReadOnlySet is recognized and relevant code gets generated.

@eiriktsarpalis
Any pointers to where I need to get started on this? A nudge to a file would be enough, I can check existing code patterns and call sites to hopefully figure the rest out.

eiriktsarpalis · 2025-10-02T08:07:11Z

Sure, you can get started by looking at how ISet<T> is being handled today, presumably IReadOnlySet<T> would get handled identically:

runtime/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs

Lines 844 to 848 in ca29ebb

    
           else if ((actualTypeToConvert = type.GetCompatibleGenericBaseType(_knownSymbols.ISetOfTType)) != null) 
        
           { 
        
               collectionType = CollectionType.ISet; 
        
               valueType = actualTypeToConvert.TypeArguments[0]; 
        
           }

Also exclude the entirety of IReadOnlySetOfTConverter by moving the #IF up.

@huoyaoyuan

…ck to csproj, which is what @huoyaoyuan (most likely) meant in his PR feedback. As far as I can see, there isn't a modern .NET identifier. Therefore, a check for .NETCoreApp is not enough, because that also includes < NET 5.0. Because of this, I also added a check that we are on .NET 5 or higher. Only then will the file be compiled.

…gh my editor says it is required

sander1095 · 2025-10-04T20:13:37Z

@eiriktsarpalis I've made changes to the source generators. Can you provide a check to see if this was done to your satisfaction? Also: If the unit tests pass, can I then assume the source generator implementation is succesful? I am not really sure how to test source generators this in the repo itself, so any tips on that would be great (if succesful unit tests aren't enough).

Thanks for your time!

src/libraries/System.Text.Json/tests/Common/JsonCreationHandlingTests.Generic.cs

…er manual testing), I believe that IReadOnlySet<T> should also not support it. TODO: Tests still need to be modified/deleted because of this change.

These tests always fail, and for good reasons. IReadOnlySet does not support CanPopulate. I verified that this is expected behavior by trying to populate IReadOnlyList, which also does not support it

I am rather sure these tests shouldn't succeed anyway. when I compare them to IReadOnlyList, (some) of these tests actually assert an exception. My goal currently is to get a successful test run, and the reviewers can tell me which tests should still be added.

…y types. Therefore, this test does not belong there.

…ssages. While ISetOfTConverter does not have this, IReadOnlyDictionaryOfTKeyTValueConverter *does*. I am not sure if there is any logic for when we do or do not include this check, but clearer exception messages seem like a plus to me.

A lot of these were part of mutable collection type tests. Previously, I added test cases for readonly collection types, and it seems like that those succeed, so I do not believe I am lowering the code coverage here.

Copilot

Pull Request Overview

Copilot reviewed 24 out of 24 changed files in this pull request and generated 2 comments.

src/libraries/System.Text.Json/tests/Common/TestClasses/TestClasses.GenericCollections.cs

...xt.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/ReferenceHandlerTests.cs

sander1095 · 2025-10-18T12:09:58Z

Hi @eiriktsarpalis and @huoyaoyuan !

I've spent quite a bit of time on this PR in the last 2 weeks. I am fairly certain support for IReadOnlySet is solid.

When I create a little test project and run JsonSerializer.Deserialize<IReadOnlySet<int>>("[1]"), the code works!

I have added tests for more in-depth coverage, but there's still some tests (especially for source generation) that fail, and I can't really figure out why. I've tried to dive deeper into these, but when I attach a debugger to the tests, nothing happens, and I've tried on WSL, Dev Container and Visual Studio.

Anyway, I believe this PR is at the point where it can get a review by the team. My goal is to obtain the following:

Some help/guidance on WHY remaining tests are failing, and how I can start approaching a fix. If the team can spare a bit of time to run this PR locally and guide me through what's going wrong, I'd really appreciate it!
Feedback on the core parts of the PR (Especially IReadOnlySetOfTConverter).
Feedback on current test coverage, and potential missing tests, or tests I should not have deleted in the last batch of commits.
- In the last commit batch I have removed several test cases because I believe they are not relevant for IReadOnlySet.

Final question:
If this PR approaches acceptability but not merge-ability, could a team member assign hacktoberfest-accepted label ;)?

huoyaoyuan

It's prioritized to finalize .NET 10 work in this month. I'm not a member of the team and can only provide auxiliary review.

huoyaoyuan · 2025-10-19T06:20:56Z

src/libraries/System.Text.Json/ref/System.Text.Json.cs

+
+        // Only modern .NET (>= 5.0) supports IReadOnlySet<T>.
+#if NET
+        public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<TCollection> CreateIReadOnlySetInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Text.Json.Serialization.Metadata.JsonCollectionInfoValues<TCollection> collectionInfo) where TCollection : System.Collections.Generic.IReadOnlySet<TElement> { throw null; }
+#endif
+


Public API can't be introduced without going through API review process. We need to go through the review like #86442 and Eirik would be responsible for this.

huoyaoyuan · 2025-10-19T06:28:41Z

src/libraries/System.Text.Json/gen/Helpers/KnownTypeSymbols.cs

+        // Only modern .NET (>= 5.0) supports IReadOnlySet<T>.
+#if NET
+        public INamedTypeSymbol? IReadOnlySetOfTType => GetOrResolveType(typeof(IReadOnlySet<>), ref _IReadOnlySetOfTType);
+        private Option<INamedTypeSymbol?> _IReadOnlySetOfTType;
+#endif
+


It's incorrect to use #if on target framework in source generator code. The generator itself can run on a different framework (.NET Framework 4.8 when using Visual Studio) than the target framework. See how Memory<T> is handled.

huoyaoyuan · 2025-10-19T06:30:34Z

src/libraries/System.Text.Json/tests/Common/CollectionTests/CollectionTests.Generic.Read.cs

            yield return new object[] { typeof(IDerivedIList) };
            yield return new object[] { typeof(IDerivedISetOfT<string>) };
+
+            // Only modern .NET (>= 5.0) supports IReadOnlySet<T>.


Nit: redundant comment; other types that only available on higher frameworks aren't commented like this.

huoyaoyuan · 2025-10-19T06:34:02Z

src/libraries/System.Text.Json/src/System.Text.Json.csproj

    <Compile Include="$(CoreLibSharedDir)System\Marvin.cs" Link="Common\System\Marvin.cs" />
  </ItemGroup>

+  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals('$(TargetFrameworkVersion)', '5.0'))">


.NET 5~7 aren't directly targeted by System.Text.Json any more. Just add it into the .NETCoreApp group, together with Int128Converter etc.

This would then include .NET Core, which did not have support for IReadOnlySet. The >= net 5.0 check is necessary for successful compilation

System.Text.Json doesn't target .NET Core 1.1 ~ 7.0 in current version. No compilation will be affected.

sander1095 added 7 commits October 1, 2025 17:13

Start adding testcases for IReadOnlySet

506bb05

This is based on existing test cases for IReadOnlyList and ISet

Add another test

419b232

Fix typo (Factorty -> Factory)

ad9f947

Add another #IF to a test case

34ec615

Start working on readonlyset converter

4f80f13

Move IReadOnlySet above ISet so ISet has priority.

5bb7f66

This seems useful for code that deserializes to a type that deserializes to a Set type that implements both interfaces.

Copilot AI review requested due to automatic review settings October 1, 2025 20:35

github-actions bot added the area-System.Text.Json label Oct 1, 2025

dotnet-policy-service bot added the community-contribution Indicates that the PR has been added by a community member label Oct 1, 2025

Copilot AI reviewed Oct 1, 2025

View reviewed changes

...xt.Json/src/System/Text/Json/Serialization/Converters/Collection/IReadOnlySetOfTConverter.cs Outdated Show resolved Hide resolved

src/libraries/System.Text.Json/tests/Common/TestClasses/TestClasses.cs Outdated Show resolved Hide resolved

sander1095 marked this pull request as draft October 1, 2025 20:37

huoyaoyuan reviewed Oct 2, 2025

View reviewed changes

...Json/src/System/Text/Json/Serialization/Converters/Collection/IEnumerableConverterFactory.cs Outdated Show resolved Hide resolved

...xt.Json/src/System/Text/Json/Serialization/Converters/Collection/IReadOnlySetOfTConverter.cs Outdated Show resolved Hide resolved

eiriktsarpalis reviewed Oct 2, 2025

View reviewed changes

sander1095 added 5 commits October 4, 2025 15:19

Change #IF pre-processors to PR suggested variant

195a155

Also exclude the entirety of IReadOnlySetOfTConverter by moving the #IF up.

Start adding support for IReadOnlySetOfT for Source Generators

6221944

Add missing newline

587f501

Remove missing newline as I do not see it on GitHub either, even thou…

2c17fa1

…gh my editor says it is required

build-analysis bot mentioned this pull request Oct 4, 2025

Intermittent build failure in AfterSourceBuild: "Could not write state file" #76488

Open

Add more code for source generator support

bdf9655

build-analysis bot mentioned this pull request Oct 4, 2025

Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. dotnet/dnceng#5922

Open

3 tasks

Add many more tests based on patterns of existing ISet tests

9f0f3fe

sander1095 added 2 commits October 4, 2025 20:35

Fix part of the build issues

c3f4662

Fix test

91a1ab0

sander1095 commented Oct 9, 2025

View reviewed changes

src/libraries/System.Text.Json/tests/Common/JsonCreationHandlingTests.Generic.cs Outdated Show resolved Hide resolved

Add whitespace

c8dc1d3

sander1095 and others added 11 commits October 9, 2025 19:13

Based on the fact that IReadOnlyList does not support population (aft…

5a54b07

…er manual testing), I believe that IReadOnlySet<T> should also not support it. TODO: Tests still need to be modified/deleted because of this change.

Delete unit tests that check for CanPopulate support.

fee078f

These tests always fail, and for good reasons. IReadOnlySet does not support CanPopulate. I verified that this is expected behavior by trying to populate IReadOnlyList, which also does not support it

Remove code that should have been removed in previous commits

ee3a3f8

Remove another test case because this list isn't filled with read onl…

6f4714b

…y types. Therefore, this test does not belong there.

Remove CanPopulate false because this is the default

deee409

Delete more test cases that I believe should fail.

78226c1

A lot of these were part of mutable collection type tests. Previously, I added test cases for readonly collection types, and it seems like that those succeed, so I do not believe I am lowering the code coverage here.

Add another test case for IReadOnlySet<T>

3052f6b

Add another test case for IReadOnlySet<T>

361f77e

Remove unintended formatting changes

ca4534d

sander1095 marked this pull request as ready for review October 18, 2025 12:04

sander1095 requested review from Copilot, eiriktsarpalis and huoyaoyuan October 18, 2025 12:05

Copilot AI reviewed Oct 18, 2025

View reviewed changes

src/libraries/System.Text.Json/tests/Common/TestClasses/TestClasses.GenericCollections.cs Outdated Show resolved Hide resolved

...xt.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/ReferenceHandlerTests.cs Show resolved Hide resolved

Sander ten Brinke added 3 commits October 18, 2025 14:18

Switch test class to implementing IReadOnlySet<T> instead of ISet<T>

d00b1e9

Add back ISet test case

309ee17

Fix .NET 5 reference in comment

3086bfc

huoyaoyuan reviewed Oct 19, 2025

View reviewed changes

Add IReadOnlySet support to System.Text.Json #120306

Are you sure you want to change the base?

Add IReadOnlySet support to System.Text.Json #120306

Uh oh!

Conversation

sander1095 commented Oct 1, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull Request Overview

Reviewed Changes

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

eiriktsarpalis left a comment

Choose a reason for hiding this comment

Uh oh!

sander1095 commented Oct 2, 2025

Uh oh!

eiriktsarpalis commented Oct 2, 2025

Uh oh!

sander1095 commented Oct 4, 2025

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull Request Overview

Uh oh!

Uh oh!

Uh oh!

sander1095 commented Oct 18, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

huoyaoyuan left a comment

Choose a reason for hiding this comment

Uh oh!

huoyaoyuan Oct 19, 2025

Choose a reason for hiding this comment

Uh oh!

huoyaoyuan Oct 19, 2025

Choose a reason for hiding this comment

Uh oh!

huoyaoyuan Oct 19, 2025

Choose a reason for hiding this comment

Uh oh!

huoyaoyuan Oct 19, 2025

Choose a reason for hiding this comment

Uh oh!

sander1095 Oct 19, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

huoyaoyuan Oct 19, 2025

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

sander1095 commented Oct 1, 2025 •

edited

Loading

sander1095 commented Oct 18, 2025 •

edited

Loading

sander1095 Oct 19, 2025 •

edited

Loading