Serializing source-generated nullable init-only properties throws when using RespectNullableAnnotations

[jupjohn]

Description

When serializing a null JSON property against a nullable init-only class property using a source generated JsonSerializerContext with RespectNullableAnnotations enabled, the serializer throws and exception claiming that the property doesn't allow null values.

This behaviour is not present when using a runtime type serializer instead of a source-generated one.

Reproduction Steps

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

const string jsonObject =
    """
    {
        "NullableProperty": null
    }
    """;

var result = JsonSerializer.Deserialize(jsonObject, NullableJsonContext.Default.MyNullableObject);
Console.WriteLine(result!.NullableProperty);


public class MyNullableObject
{
    public string? NullableProperty { get; init; }
}

// Setting RespectNullableAnnotations to false will make this pass, as will changing the init accessor to a private setter
[JsonSourceGenerationOptions(RespectNullableAnnotations = true)]
[JsonSerializable(typeof(MyNullableObject))]
public partial class NullableJsonContext : JsonSerializerContext;

Expected behavior

The nullable property (NullableProperty in the example) serializes the property as a null value.

Actual behavior

Serialization throws a JsonException:

Unhandled exception. System.Text.Json.JsonException: The constructor parameter 'NullableProperty' on type 'MyNullableObject' doesn't allow null values. Consider updating its nullability annotation. Path: $.NullableProperty | LineNumber: 1 | BytePositionInLine: 28.
   at System.Text.Json.ThrowHelper.ThrowJsonException_ConstructorParameterDisallowNull(String parameterName, Type declaringType)
   at System.Text.Json.Serialization.Converters.LargeObjectWithParameterizedConstructorConverter`1.ReadAndCacheConstructorArgument(ReadStack& state, Utf8JsonReader& reader, JsonParameterInfo jsonParameterInfo)
   at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.Converters.JsonMetadataServicesConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value, Boolean& isPopulatedValue)
   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, T& value, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.Deserialize(Utf8JsonReader& reader, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 utf8Json, JsonTypeInfo`1 jsonTypeInfo, Nullable`1 actualByteCount)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo`1 jsonTypeInfo)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonTypeInfo`1 jsonTypeInfo)
   at Program.<Main>$(String[] args) in /.../Program.cs:line 11

Regression?

No response

Known Workarounds

Using a non-source generated serializer works:

JsonSerializer.Deserialize<MyNullableObject>(jsonObject);

Configuration

Tested on:

• .NET 8
• System.Text.Json 9.0.0-rc.1.24431.7
• Windows & Linux
• x64
• Platform/arch independent

Other information

No response

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