System.Text.Json fast path serialization not working with combined contexts #71933
Description
EDIT: See API proposal at the bottom of this post.
The internal logic governing use of fast path serialization is dependent on JsonSerializerOptions encapsulating a JsonSerializerContext instance. Consider:
or
or
The introduction of contract customization has made it possible for source generated metadata to be associated with options instances not attached to a JsonSerializerContext. This can result in inconsistency, consider:
[JsonSourceGenerationOptions(GenerationMode = JsonSourceGenerationMode.Serialization)]
[JsonSerializable(typeof(HighLowTemps))]
public partial class SerializationContext : JsonSerializerContext {}
public class HighLowTemps
{
public int High { get; set; }
public int Low { get; set; }
}
[Fact]
public static void CombiningContexts_FastPathSerialization()
{
var value = new HighLowTemps { High = 40, Low = 30 };
string expectedJson = """{"High":40,"Low":30}""";
// Sanity check -- default context
var options = new JsonSerializerOptions { TypeInfoResolver = SerializationContext.Default };
string json = JsonSerializer.Serialize(value, options);
Assert.Equal(expectedJson, json);
// Using a trivial combined context
options = new JsonSerializerOptions { TypeInfoResolver = JsonTypeInfoResolver.Combine(SerializationContext.Default) };
json = JsonSerializer.Serialize(value, options); // 'JsonSerializerContext' '<null>' did not provide property metadata
// for type 'System.Text.Json.SourceGeneration.Tests.HighLowTemps'.
Assert.Equal(expectedJson, json);
}In order to get the issue fixed, it is necessary for source gen JsonTypeInfo<T> to encapsulate a reference to their originating JsonSerializerContext.
API Proposal
namespace System.Text.Json.Serialization.Metadata;
public class JsonTypeInfo
{
public IJsonTypeInfoResolver? OriginatingResolver { get; set; } = null;
}