NullReferenceException when deserializing object containing nullable struct using source generator

[wt-brett]

Description

Calling JsonSerializer.Deserialize for an object containing a nullable struct property will throw a NullReferenceException when passing JsonTypeInfo.

Reproduction Steps

1. Create a new Console App Project.
2. Use the following code:

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

var obj = new MyObj { MyStruct = new MyStruct { Property1 = 2 } };
var json = JsonSerializer.Serialize(obj, SerializerContext.Default.MyObj);
var result = JsonSerializer.Deserialize(json, SerializerContext.Default.MyObj);

public record class MyObj
{
    public MyStruct? MyStruct { get; init; }
}

public struct MyStruct
{
    public byte Property1 { get; init; }
}

[JsonSerializable(typeof(MyObj))]  
internal partial class SerializerContext : JsonSerializerContext
{
}

A NullReferenceException will be thrown on the var result = JsonSerializer.Deserialize(json, SerializerContext.Default.MyObj); line

Expected behavior

Calling JsonSerializer.Deserialize method does not throw NullReferenceException

Actual behavior

Exception Message:
Object reference not set to an instance of an object.

Stack Trace:

at System.Text.Json.Serialization.Converters.LargeObjectWithParameterizedConstructorConverter`1.InitializeConstructorArgumentCaches(ReadStack& state, JsonSerializerOptions options)
 at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`1.BeginRead(ReadStack& state, JsonSerializerOptions options)
 at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`1.ReadConstructorArguments(ReadStack& state, Utf8JsonReader& reader, JsonSerializerOptions options)
 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.Converters.NullableConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, Nullable`1& 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.TryReadAsObject(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, Object& value)
 at System.Text.Json.Serialization.Converters.LargeObjectWithParameterizedConstructorConverter`1.ReadAndCacheConstructorArgument(ReadStack& state, Utf8JsonReader& reader, JsonParameterInfo jsonParameterInfo)
 at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`1.ReadConstructorArguments(ReadStack& state, Utf8JsonReader& reader, JsonSerializerOptions options)
 at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`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, JsonSerializerOptions options, 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)

Regression?

Yes. This code worked correctly in .net 8 preview 3

Known Workarounds

Call var result = JsonSerializer.Deserialize<MyObj>(json); without using the JsonTypeInfo

Configuration

• Which version of .NET is the code running on?
  .net 8 preview 4

• What OS and version, and what distro if applicable?
  Windows 11

• What is the architecture (x64, x86, ARM, ARM64)?
  x64

• Do you know whether it is specific to that configuration?
  Not sure

Other information

No response

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

Issue Details

---

Description

Calling JsonSerializer.Deserialize for an object containing a nullable struct property will throw a NullReferenceException when passing JsonTypeInfo.

Reproduction Steps

1. Create a new Console App Project.
2. Use the following code:

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

var obj = new MyObj { MyStruct = new MyStruct { Property1 = 2 } };
var json = JsonSerializer.Serialize(obj, SerializerContext.Default.MyObj);
var result = JsonSerializer.Deserialize(json, SerializerContext.Default.MyObj);

public record class MyObj
{
    public MyStruct? MyStruct { get; init; }
}

public struct MyStruct
{
    public byte Property1 { get; init; }
}

[JsonSerializable(typeof(MyObj))]  
internal partial class SerializerContext : JsonSerializerContext
{
}

A NullReferenceException will be thrown on the var result = JsonSerializer.Deserialize(json, SerializerContext.Default.MyObj); line

Expected behavior

Calling JsonSerializer.Deserialize method does not throw NullReferenceException

Actual behavior

Exception Message:
Object reference not set to an instance of an object.

Stack Trace:

at System.Text.Json.Serialization.Converters.LargeObjectWithParameterizedConstructorConverter`1.InitializeConstructorArgumentCaches(ReadStack& state, JsonSerializerOptions options)
 at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`1.BeginRead(ReadStack& state, JsonSerializerOptions options)
 at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`1.ReadConstructorArguments(ReadStack& state, Utf8JsonReader& reader, JsonSerializerOptions options)
 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.Converters.NullableConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, Nullable`1& 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.TryReadAsObject(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, Object& value)
 at System.Text.Json.Serialization.Converters.LargeObjectWithParameterizedConstructorConverter`1.ReadAndCacheConstructorArgument(ReadStack& state, Utf8JsonReader& reader, JsonParameterInfo jsonParameterInfo)
 at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`1.ReadConstructorArguments(ReadStack& state, Utf8JsonReader& reader, JsonSerializerOptions options)
 at System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter`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, JsonSerializerOptions options, 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)

Regression?

Yes. This code worked correctly in .net 8 preview 3

Known Workarounds

Call var result = JsonSerializer.Deserialize<MyObj>(json); without using the JsonTypeInfo

Configuration

• Which version of .NET is the code running on?
  .net 8 preview 4

• What OS and version, and what distro if applicable?
  Windows 11

• What is the architecture (x64, x86, ARM, ARM64)?
  x64

• Do you know whether it is specific to that configuration?
  Not sure

Other information

No response

Author:	wt-brett
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[eiriktsarpalis]

It looks like the Nullable converter is not forwarding relevant metadata of the converter it is encapsulating. Minimal reproduction:

JsonSerializer.Deserialize<MyStruct?>("""{"Value":42}""");

public readonly struct MyStruct
{
    [JsonConstructor]
    public MyStruct(int value) => Value = value;
    public int Value { get; }
}