[Bug]: Serializer.SerializeResponseAsync writes synchronously to the target stream #399

What type of issue is this?

• Bug report
• Feature request

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the SysML2.NET
• I have searched open and closed issues to ensure it has not already been reported

Description

Both Serializer.SerializeResponseAsync overloads (IResponse and IEnumerable<IResponse>) perform a synchronous write on the target stream before their await writer.FlushAsync(...): the private WriteSingle and WriteMany helpers end with writer.Flush(), which calls Stream.Write.

On a stream that disallows synchronous I/O — ASP.NET Core's response body under Kestrel or TestServer, where AllowSynchronousIO is false by default — this throws:

System.InvalidOperationException: Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true.
   at System.IO.Stream.Write(ReadOnlySpan`1 buffer)
   at System.Text.Json.Utf8JsonWriter.Flush()
   at SysML2.NET.Serializer.Json.Serializer.WriteMany(...)
   at SysML2.NET.Serializer.Json.Serializer.SerializeResponseAsync(IEnumerable`1 dataItems, ...)

Since the response status has usually been sent by then, an HTTP server answers 200 with an empty body. The SerializeAsync overloads (Core elements) are not affected: they use FlushAsync throughout.

Expected: the …Async methods perform no synchronous I/O on the target stream.

Suggested fix: remove writer.Flush() from WriteSingle / WriteMany and keep the trailing await writer.FlushAsync(cancellationToken) in the async callers; the synchronous SerializeResponse overloads then flush after calling the helpers.

Steps to Reproduce

1. Host a minimal ASP.NET Core endpoint that writes a PSM response straight to HttpResponse.Body:

   await serializer.SerializeResponseAsync(new[] { new Project { Id = Guid.NewGuid() } }, SerializationModeKind.JSON, false, context.Response.Body, new JsonWriterOptions(), context.RequestAborted);

2. Call it through Kestrel or WebApplicationFactory / TestServer.
3. The call throws InvalidOperationException; the client receives an empty body.

A unit test can reproduce it without a host by passing a Stream whose synchronous Write throws (e.g. a Mock<Stream> that only implements WriteAsync).

System Configuration

• SysML2.NET version: 0.25.1 (SysML2.NET.Serializer.Json)
• Environment (Operating system, version and so on): Windows 11, ASP.NET Core 10 (Kestrel / TestServer)
• .NET Framework version: .NET 10
• Additional information: Mycelium Fabric works around it by serializing into a MemoryStream and copying it to the response with CopyToAsync.