[STJ Source gen] Use UnsafeAccessor/reflection for inaccessible members

[Copilot]

Description

Fixes #84484
Fixes #88519
Fixes #107898

The source generator uses [UnsafeAccessor] (NET8+ non-generic types) or cached reflection delegates (older targets/generic types) to access inaccessible members, enabling three previously unsupported scenarios:

1. Preserve default values for init-only properties

The source generator previously set init-only properties via the constructor delegate's object initializer expression, even when absent from JSON. This replaced C# property initializer defaults with default(T):

public class Foo
{
    public string Name { get; init; } = "DefaultName";
    public int Number { get; init; } = 42;
}
// Previously: static args => new Foo() { Name = (string)args[0], Number = (int)args[1] }
// Deserializing "{}" gave Name=null, Number=0 instead of Name="DefaultName", Number=42

Init-only properties are now removed from the constructor delegate and set post-construction via UnsafeAccessor or reflection, only when present in JSON.

2. Enable [JsonInclude] for inaccessible properties

Previously, [JsonInclude] properties with inaccessible getters or setters generated throwing delegates. They are now fully supported:

public class Bar
{
    [JsonInclude]
    private string Secret { get; set; } = "hidden";
}

3. Enable [JsonConstructor] for inaccessible constructors

Previously, inaccessible constructors marked with [JsonConstructor] were ignored with a SYSLIB1222 warning. They are now invoked via [UnsafeAccessor(UnsafeAccessorKind.Constructor)] or cached ConstructorInfo.Invoke reflection fallback:

public class Baz
{
    [JsonConstructor]
    private Baz(string name) { Name = name; }
    public string Name { get; }
}

---

Generated Code Samples

UnsafeAccessor path (NET8+ non-generic types)

For a type with a private [JsonInclude] property and an init-only property:

public class MyType
{
    [JsonInclude]
    private string Secret { get; set; } = "default";

    public int Value { get; init; } = 42;
}

The source generator emits [UnsafeAccessor] extern methods that are invoked directly from the getter/setter delegates. JsonPropertyInfoValues<T> is generic on the property type, so the Setter delegate is Action<object, T?> and the Getter is Func<object, T?>. The only cast in each delegate is on the object parameter to the declaring type:

Property getter (inaccessible [JsonInclude])

[UnsafeAccessor(UnsafeAccessorKind.Method, Name = "get_Secret")]
private static extern string __get_MyType_Secret(MyType obj);

// Delegate: Func<object, string?>
Getter = static obj => __get_MyType_Secret((MyType)obj),

Property setter (inaccessible [JsonInclude])

[UnsafeAccessor(UnsafeAccessorKind.Method, Name = "set_Secret")]
private static extern void __set_MyType_Secret(MyType obj, string value);

// Delegate: Action<object, string?>
Setter = static (obj, value) => __set_MyType_Secret((MyType)obj, value!),

Init-only property setter

[UnsafeAccessor(UnsafeAccessorKind.Method, Name = "set_Value")]
private static extern void __set_MyType_Value(MyType obj, int value);

// Delegate: Action<object, int?>
Setter = static (obj, value) => __set_MyType_Value((MyType)obj, value!),

Constructor (inaccessible [JsonConstructor])

[UnsafeAccessor(UnsafeAccessorKind.Constructor)]
private static extern Baz __ctor_Baz(string p0);

// Invoked directly from the parameterized constructor delegate

Value types (structs)

For struct types, the extern uses ref and the delegate passes ref Unsafe.Unbox<T>(obj):

[UnsafeAccessor(UnsafeAccessorKind.Method, Name = "set_Value")]
private static extern void __set_MyStruct_Value(ref MyStruct obj, int value);

Setter = static (obj, value) => __set_MyStruct_Value(ref Unsafe.Unbox<MyStruct>(obj), value!),

Reflection fallback (older targets or generic types)

When [UnsafeAccessor] is not available (e.g., netstandard2.0 targets) or the declaring type is generic (not supported by UnsafeAccessor), the generator falls back to cached reflection with strongly typed wrappers:

Property getter (reflection)

private static Func<MyType, string>? s_get_MyType_Secret;
private static string __get_MyType_Secret(MyType obj) =>
    (s_get_MyType_Secret ??= (Func<MyType, string>)Delegate.CreateDelegate(
        typeof(Func<MyType, string>),
        typeof(MyType).GetProperty("Secret", BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)!
            .GetGetMethod(true)!))((MyType)obj);

Getter = static obj => __get_MyType_Secret((MyType)obj),

Property setter (reflection)

private static Action<MyType, string>? s_set_MyType_Secret;
private static void __set_MyType_Secret(MyType obj, string value) =>
    (s_set_MyType_Secret ??= (Action<MyType, string>)Delegate.CreateDelegate(
        typeof(Action<MyType, string>),
        typeof(MyType).GetProperty("Secret", BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)!
            .GetSetMethod(true)!))((MyType)obj, value);

Setter = static (obj, value) => __set_MyType_Secret((MyType)obj, value!),

Field accessor (reflection)

For fields, FieldInfo is cached directly since fields don't have MethodInfo equivalents:

private static FieldInfo? s_field_MyType_myField;
private static string __get_MyType_myField(object obj) =>
    (string)(s_field_MyType_myField ??= typeof(MyType).GetField("myField",
        BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)!).GetValue(obj)!;

private static void __set_MyType_myField(object obj, string value) =>
    (s_field_MyType_myField ??= typeof(MyType).GetField("myField",
        BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)!).SetValue(obj, value);

Constructor (reflection)

Note: Unlike MethodInfo, ConstructorInfo cannot be wrapped in a delegate, so the ConstructorInfo itself is cached:

private static ConstructorInfo? s_ctor_Baz;
private static Baz __ctor_Baz(string p0) =>
    (Baz)(s_ctor_Baz ??= typeof(Baz).GetConstructor(
        BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic,
        binder: null, new Type[] { typeof(string) }, modifiers: null)!)
    .Invoke(new object?[] { p0 });

---

Changes

• Parser: Non-required init-only properties removed from PropertyInitializerSpecs (renamed memberInitializerNames to requiredMemberNames). Inaccessible [JsonInclude] properties no longer flagged as HasInvalidConfigurationForFastPath. Inaccessible [JsonConstructor] constructors no longer nulled out ΓÇö tracked as inaccessible for the emitter.

• PropertyGenerationSpec: Added CanUseUnsafeAccessors (per-property, checks UnsafeAccessorAttribute availability and non-generic declaring type), NeedsAccessorForGetter, and NeedsAccessorForSetter.

• TypeGenerationSpec: Added ConstructorIsInaccessible and CanUseUnsafeAccessorForConstructor.

• Emitter: Unified accessor pattern with identical wrapper signatures for both paths. UnsafeAccessor externs are implementation details. Struct types use ref Unsafe.Unbox<T>(obj). Fast-path serialization includes inaccessible [JsonInclude] properties. Field reflection fallback correctly caches FieldInfo and calls .GetValue() / .SetValue().

• KnownTypeSymbols: Detect UnsafeAccessorAttribute availability.

• Runtime validation (JsonMetadataServices.Helpers.cs): Allow non-public [JsonInclude] properties when getter/setter delegates are provided.

• Diagnostics: SYSLIB1038 and SYSLIB1222 no longer emitted for these scenarios.

• Tests: Updated overrides, added new test types and methods including generic type coverage for the reflection fallback path (GenericClassWithPrivateJsonIncludeProperties<T>). All 7805 source gen tests pass.

---

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[eiriktsarpalis]

@copilot Let's try to broaden the scope of this PR a little bit. We can also use unsafe accessors (or the reflection fall-back) to finally enable JsonIncludeAttribute support in the source generator. If a member is marked JsonInclude but is not accessible by the current generation context, we instruct the source generator to emit getter and setter delegates using unsafe accessors.

[Copilot]

@copilot Let's try to broaden the scope of this PR a little bit. We can also use unsafe accessors (or the reflection fall-back) to finally enable JsonIncludeAttribute support in the source generator...

Broadened scope — [JsonInclude] properties with inaccessible getters/setters now generate real accessor delegates via [UnsafeAccessor] (when available) or Lazy<PropertyInfo> reflection fallback, instead of throwing. The InaccessibleJsonIncludePropertiesNotSupported diagnostic is suppressed since these members are now supported. See ccad3dd.

[Copilot]

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[eiriktsarpalis]

@copilot please add extensive tests for [JsonInclude] against private/inaccessible members in the source generator.

[Copilot]

@copilot please add extensive tests for [JsonInclude] against private/inaccessible members in the source generator.

Added extensive tests in 7b5c98e:

Updated existing overrides — all source gen test methods that previously expected InvalidOperationException for inaccessible [JsonInclude] members now delegate to base tests (which expect success):

• Honor_JsonSerializablePropertyAttribute_OnProperties
• NonPublicInitOnlySetter_With_JsonInclude (all access levels)
• HonorCustomConverter_UsingPrivateSetter
• Public_And_NonPublicPropertyAccessors_PropertyAttributes
• HonorJsonPropertyName_PrivateGetter/PrivateSetter
• NonPublicProperty_JsonInclude_WorksAsExpected (all 9 type/accessibility combinations)

New test types: ClassWithPrivateJsonIncludeProperties_Roundtrip, ClassWithProtectedJsonIncludeProperties_Roundtrip, ClassWithMixedAccessibilityJsonIncludeProperties, ClassWithJsonIncludePrivateInitOnlyProperties, ClassWithJsonIncludePrivateGetterProperties, StructWithJsonIncludePrivateProperties

New test methods: roundtrip for private properties, roundtrip for protected properties, mixed accessibility (public/internal/private/protected), private init-only with default preservation, private getter serialization, empty JSON deserialization, struct with private properties.

[eiriktsarpalis]

@copilot please address the most recent merge conflicts

[Copilot]

@copilot please address the most recent merge conflicts

Merged with main and resolved all conflicts in f7275aa. Key resolutions:

• Parser.cs: Accepted main's typeNamingPolicy / JsonKnownNamingPolicy additions (6 conflict regions)
• DefaultJsonTypeInfoResolver.Helpers.cs: Accepted main's typeNamingPolicy parameter additions
• PropertyVisibilityTests.cs: Kept our await base.X() delegation (inaccessible members now supported)
• SourceGeneration.Tests.targets: Kept SYSLIB1038/SYSLIB1222 excluded (no longer emitted)
• Non-STJ files (gc.cpp, crsttypes_generated.h, etc.): took main's version

[eiriktsarpalis]

@copilot there still appear to be merge conflicts.

[Copilot]

@copilot there still appear to be merge conflicts.

Resolved in fc6bdd2. Two conflicts from main's latest changes:

• PropertyVisibilityTests.cs: kept our await base.X() delegation (inaccessible [JsonInclude] now supported)
• Tests.targets: kept SYSLIB1038/SYSLIB1222 excluded (no longer emitted)

[github-actions]

Code Review: System.Text.Json Source Generation — UnsafeAccessor / Reflection for Inaccessible Members

Note

This review was generated by Copilot.

Summary

Two commits: (1) adds UnsafeAccessor/reflection fallback for inaccessible members, init-only properties, and inaccessible constructors; (2) temporarily disables [JsonInclude] for inaccessible members by setting HasJsonInclude = false. The init-only property and inaccessible-constructor features remain active.

---

❌ Field setter reflection fallback silently drops writes on value types (latent bug)

File: Emitter.cs ~L867-873, ~L996-999

GetAccessorBasedSetterDelegate generates the same caller shape for both property and field setters on value types:

static (obj, value) => __set_Type_Field(ref Unsafe.Unbox<T>(obj), value!)

But the field wrapper takes (object obj, ...) (line 999), not (ref T obj, ...). When called, the ref is dereferenced and re-boxed into a fresh object; FieldInfo.SetValue mutates that copy, not the original. The property wrapper correctly takes (ref T, ...) so it works fine.

Currently unreachable because commit 2 sets HasJsonInclude = false for inaccessible members, so NeedsAccessorForSetter returns false via the !CanUseSetter && HasJsonInclude branch. Fields can never be init-only, so the other branch doesn't apply. However, the TODO at Parser.cs L1379 explicitly plans to re-enable this. When re-enabled, this will silently corrupt struct deserialization for generic types with inaccessible [JsonInclude] fields.

Fix: In GetAccessorBasedSetterDelegate, when !CanUseUnsafeAccessors && !IsProperty, the wrapper has signature (object, TValue) — the caller should pass obj directly (not ref Unsafe.Unbox), or better: generate the field wrapper with (ref T, TValue) signature to match the property pattern.

---

❌ Baseline unit tests not updated after commit 2

File: JsonSourceGeneratorOutputTests.cs L258-275 (UnsafeAccessors_PrivateProperties)

The UnsafeAccessors_PrivateProperties baseline test asserts generated code containing HasJsonInclude = true and UnsafeAccessor externs for private properties. Commit 2 sets HasJsonInclude = false for inaccessible members, so the generator now produces code that does not match these baselines. The test was not disabled or updated.

The commit message for commit 2 claims it "Skip[s] affected source gen tests with [ActiveIssue]" but the actual diff only touches 3 files and does not add any [ActiveIssue] attributes or update any baselines. This will cause unit test failures.

---

⚠️ propertyInitializerSpecs skipped for inaccessible constructors — required members not in ctor delegate

File: Parser.cs ~L694-698

When constructorIsInaccessible is true, ParsePropertyInitializers is skipped entirely. Previously (old code) the constructor was set to null so it fell back to the default constructor and property initializers were generated for required members. Now the inaccessible parameterized constructor is used directly, but required member initializers (those that don't match ctor params) are dropped from the delegate. These required members would only be set through their normal setter delegates, losing the deserialization-time enforcement that the initializer list provides.

This is acceptable if the STJ deserializer independently enforces required semantics (which it does), but it's a subtle behavior change worth calling out. The InaccessibleCtor baseline confirms this: property setters are null (get-only props), and all values come through ctor params.

---

💡 Static reflection cache fields with ??= — benign race (no issue)

File: Emitter.cs ~L951, 957, 971-972, 977-978

The private static Func<>? s_get_... and similar cache fields are lazily initialized with ??=. Multiple threads may redundantly compute the same value, but Delegate.CreateDelegate / GetProperty always produces equivalent results. The worst case is a few extra reflective lookups during initial concurrent access. No correctness or thread-safety issue.

---

💡 Init-only property handling change — intentional default-value preservation (no issue)

File: Parser.cs ~L1714-1718, Emitter.cs ~L818-821

Previously, init-only properties were included as property initializers in the constructor delegate:

static args => new T() { InitProp = (T)args[N] }

When a property was absent from JSON, args[N] was default(T), overwriting the C# default. Now, init-only properties get UnsafeAccessor-based setters and are set only when present in JSON, preserving defaults. This is a correctness improvement, well-tested (InitOnlyPropertyDefaultValues_PreservedWhenMissingFromJson). This is a behavioral change for existing users who may have relied on the prior (arguably buggy) behavior.

---

💡 Reflection GetValue/SetValue for fields on value types — getter is fine (no issue for reads)

File: Emitter.cs ~L990-993

The field getter wrapper passes object to FieldInfo.GetValue(), which reads the field from the boxed struct. Even with the unnecessary unbox-rebox cast from the caller, the read is correct because it returns a copy of the field value.

---

💡 Delegate.CreateDelegate for value type property setters — correct (no issue)

File: Emitter.cs ~L966-972

The custom ValueTypeSetter<TDeclaringType, TValue> delegate type takes ref TDeclaringType, allowing Delegate.CreateDelegate to bind to the property setter's MethodInfo with the struct passed by reference. The caller invokes via ref Unsafe.Unbox<T>(obj), mutating the boxed struct in-place. This pattern is correct and consistent with existing Unsafe.Unbox usage in the codebase.

---

💡 Inaccessible constructors + UnsafeAccessor — correct (no issue)

File: Emitter.cs ~L1058-1110, Parser.cs ~L681-698

The constructor is no longer nulled out when inaccessible. Instead, ConstructorIsInaccessible drives emission of either a [UnsafeAccessor(Constructor)] extern or a cached ConstructorInfo.Invoke fallback. The generated baseline code (confirmed in MyContext.InaccessibleCtor.g.cs.txt) is correct.

---

Summary Table

Item	Severity	Status
Field setter reflection on value types	❌	Latent bug — will surface when TODO at L1379 is resolved
Baseline tests not updated for commit 2	❌	UnsafeAccessors_PrivateProperties baseline will fail
Required members with inaccessible ctor	⚠️	Works via STJ enforcement, but initializer list is dropped
??= cache thread safety	💡	Benign race, no issue
Init-only default preservation	💡	Correct behavioral improvement
Field getter boxing	💡	Correct for reads
Value type setter delegate	💡	Correct
Inaccessible constructor accessor	💡	Correct

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