System.Text.Json source generator emits duplicate __GenericAccessors_ wrapper class for inaccessible members inherited from multiple generic base types

[svick]

Description

The System.Text.Json source generator emits colliding helper classes for inaccessible members (e.g. a private [JsonInclude] property) that are inherited from two different generic base types of the same arity. The generated code then fails to compile with an error like CS0102: The type 'MyContext' already contains a definition for '__GenericAccessors_Leaf' (plus possibly a cascade of CS0264/CS0265/CS0699/CS0246).

This is a compilation failure that only happens in .NET 11; the same program compiles on .NET 10 (with a SYSLIB1038 warning), but then throws an InvalidOperationException when you attempt to use the generated serializer context.

Repro

Program.cs:

using System.Text.Json;
using System.Text.Json.Serialization;

Console.WriteLine(JsonSerializer.Serialize(new Leaf(), MyContext.Default.Leaf));

public class GrandBase<TG> where TG : class
{
    [JsonInclude]
    private TG GrandSecret { get; set; } = default!;
}

public class Base<TB> : GrandBase<string> where TB : struct
{
    [JsonInclude]
    private TB BaseSecret { get; set; }
}

public class Leaf : Base<int> { }

[JsonSerializable(typeof(Leaf))]
internal partial class MyContext : JsonSerializerContext { }

hwapp.csproj (multi-targeted to show the difference):

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>net10.0;net11.0</TargetFrameworks>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>
</Project>

Behavior change between .NET 10 and .NET 11

Under .NET 10 the source generator does not emit accessors for inaccessible members: it reports SYSLIB1038 and the type throws InvalidOperationException at run time. Under .NET 11 the generator now emits [UnsafeAccessor] accessor wrapper classes for inaccessible members (added in #124650), but for members inherited from two different generic base types of the same arity the wrapper classes collide and the generated code no longer compiles.

> dotnet run -f net10.0
C:\code\tmp\hwapp\Program.cs(15,16): warning SYSLIB1038: The member 'Base.BaseSecret' has been annotated with the JsonIncludeAttribute but is not visible to the source generator. (https://learn.microsoft.com/dotnet/fundamentals/syslib-diagnostics/syslib1038)
C:\code\tmp\hwapp\Program.cs(9,16): warning SYSLIB1038: The member 'GrandBase.GrandSecret' has been annotated with the JsonIncludeAttribute but is not visible to the source generator. (https://learn.microsoft.com/dotnet/fundamentals/syslib-diagnostics/syslib1038)
Unhandled exception. System.InvalidOperationException: The property 'BaseSecret' on type 'Base`1[System.Int32]' which is annotated with 'JsonIncludeAttribute' is not accesible by the source generator.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_JsonIncludeOnInaccessibleProperty(String memberName, Type declaringType)
   at System.Text.Json.Serialization.Metadata.JsonMetadataServices.PopulateProperties(JsonTypeInfo typeInfo, JsonPropertyInfoList propertyList, Func`2 propInitFunc)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<get_PropertyList>g__CreatePropertyList|63_0()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.ConfigureProperties()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<EnsureConfigured>g__ConfigureSynchronized|174_0()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.JsonSerializerOptions.GetTypeInfo(Type type)
   at MyContext.get_Leaf() in ...\MyContext.Leaf.g.cs:line 20
   at Program.<Main>$(String[] args) in C:\code\tmp\hwapp\Program.cs:line 4

> dotnet run -f net11.0
...\MyContext.Leaf.g.cs(121,26): error CS0102: The type 'MyContext' already contains a definition for '__GenericAccessors_Leaf'
...\MyContext.Leaf.g.cs(113,26): error CS0264: Partial declarations of 'MyContext.__GenericAccessors_Leaf<TB>' must have the same type parameter names in the same order
...\MyContext.Leaf.g.cs(121,60): error CS0699: 'MyContext.__GenericAccessors_Leaf<TB>' does not define type parameter 'TG'
...\MyContext.Leaf.g.cs(113,26): error CS0265: Partial declarations of 'MyContext.__GenericAccessors_Leaf<TB>' have inconsistent constraints for type parameter 'TB'
...\MyContext.Leaf.g.cs(124,74): error CS0246: The type or namespace name 'TG' could not be found (are you missing a using directive or an assembly reference?)
...\MyContext.Leaf.g.cs(124,30): error CS0246: The type or namespace name 'TG' could not be found (are you missing a using directive or an assembly reference?)
...\MyContext.Leaf.g.cs(126,76): error CS0246: The type or namespace name 'TG' could not be found (are you missing a using directive or an assembly reference?)
...\MyContext.Leaf.g.cs(126,85): error CS0246: The type or namespace name 'TG' could not be found (are you missing a using directive or an assembly reference?)

The build failed. Fix the build errors and run again.

(Reproduced on SDK 11.0.100-rc.2.26431.116; the net10.0 run used the .NET 10 GA shared framework.)

Root cause

The generator emits one member-accessor wrapper class per declaring type (keyed by property.DeclaringType.FullyQualifiedName), but names each one only after the constructed type:

// JsonSourceGenerator.Emitter.cs
writer.WriteLine($"private static class __GenericAccessors_{typeFriendlyName}<{typeParamList}>{constraintClauses}");

Here typeFriendlyName is the constructed type's TypeInfoPropertyName and typeParamList/constraintClauses come from the declaring type. When a type has inaccessible members inherited from two different generic base types of the same arity, both wrappers are emitted as __GenericAccessors_Leaf<...> with arity 1, so C# sees the same type defined twice.

Proposed fix

Include a declaring-type discriminator in the wrapper class name so each declaring type gets a distinct wrapper (analogous to how the per-member accessor methods are already disambiguated).

Notes

• Closely related to System.Text.Json source generator emits invalid constructor accessor for generic types with inaccessible constructors (MissingMethodException) #133369 (fixed by Fix System.Text.Json source generator constructor accessor for generic types with inaccessible constructors #133406), which is the same class of naming problem on the constructor accessor path; this issue is the member accessor path and is separate.
• The different type parameter names and constraints are not not necessary to reproduce the issue, but show that each base class requires a separate wrapper class.

Note

This issue (including the reproduced output) was created with the help of GitHub Copilot.

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[jeffhandley]

@eiriktsarpalis - If we have time to fix this in .NET 11, it would have my support in RC2 or GA, but I wouldn't consider it release-blocking as we could also address this in servicing.

Thanks for reporting it, @svick!