Using the System.Text.Json source generator with ASP NET Core throws NotSupportedException when attempting to serialize IEnumerable<T>

[JakeYallop]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

This was a scenario that was working in .NET 6.0.

When using the .NET 7.0 System.Text.Json source generator with controllers (this is not reproduceable using minimal APIs), ASP NET Core fails to deserialize IEnumerable results.

NotSupportedException: Metadata for type 'System.Collections.Generic.List`1[JsonSourceGenIEnumerableRepro.MyClass]' was not provided by TypeInfoResolver of type 'JsonSourceGenIEnumerableRepro.JsonContext'. If using source generation, ensure that all root types passed to the serializer have been indicated with 'JsonSerializableAttribute', along with any types that might be serialized polymorphically.
  System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType(Type type, IJsonTypeInfoResolver resolver)
  System.Text.Json.JsonSerializerOptions.GetTypeInfoCached(Type type)
  System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type runtimeType)
  System.Text.Json.JsonSerializer.SerializeAsync(Stream utf8Json, object value, Type inputType, JsonSerializerOptions options, CancellationToken cancellationToken)
  Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
  Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeResultFilters>g__Awaited|28_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, object state, bool isCompleted)
  Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|20_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, object state, bool isCompleted)
  Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
  Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
  Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
  Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

Calling JsonSerializer.Serialize(enumerable, JsonContext.Default.IEnumerableJsonTypeInfo) manually succeeds.

Expected Behavior

JSON serialization by the ASP NET Core controller infrastructure has the same behaviour as if JsonSerializer.Serialize was called.

Steps To Reproduce

https://github.com/JakeYallop/JsonSourceGeneratorIEnumerableRepro

using Microsoft.AspNetCore.Mvc;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace JsonSourceGenIEnumerableRepro;

public class Program
{
    public static void Main(string[] args)
    {
        var builder = WebApplication.CreateBuilder(args);

        var l = new List<MyClass> { new(), new() };
        //using the STJ source gen default infrastructure succeeds without failing
        var output = JsonSerializer.Serialize(l, JsonContext.Default.IEnumerableMyClass);

        builder.Services
            .AddMvcCore()
            .AddJsonOptions(options =>
            {
                options.JsonSerializerOptions.AddContext<JsonContext>();
            });


        var app = builder.Build();

        app.MapControllers();
        app.Run();
    }
}


[Route("")]
public class MyController : ControllerBase
{
    public MyController()
    {
    }

    [HttpGet("")]
    public IActionResult Get()
    {
        var l = new List<MyClass> { new(), new() };
        return Ok(l);
    }
}

[JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
[JsonSerializable(typeof(IEnumerable<MyClass>))]
public partial class JsonContext : JsonSerializerContext
{

}

public class MyClass
{
    public int MyProp { get; set; }
}

Exceptions (if any)

See above

.NET Version

7.0.100-rc.2.22459.2

Anything else?

dotnet --info output (click to view)

``` .NET SDK: Version: 7.0.100-rc.2.22459.2 Commit: 11f6b8f712

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19043
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\7.0.100-rc.2.22459.2\

Host:
Version: 7.0.0-rc.2.22451.11
Architecture: x64
Commit: 6d10e4c8bc

.NET SDKs installed:
3.1.422 [C:\Program Files\dotnet\sdk]
5.0.100 [C:\Program Files\dotnet\sdk]
5.0.104 [C:\Program Files\dotnet\sdk]
5.0.214 [C:\Program Files\dotnet\sdk]
5.0.303 [C:\Program Files\dotnet\sdk]
6.0.101 [C:\Program Files\dotnet\sdk]
6.0.108 [C:\Program Files\dotnet\sdk]
6.0.202 [C:\Program Files\dotnet\sdk]
6.0.203 [C:\Program Files\dotnet\sdk]
6.0.400 [C:\Program Files\dotnet\sdk]
7.0.100-rc.2.22459.2 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.10 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 7.0.0-rc.2.22452.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 7.0.0-rc.2.22451.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.28 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 7.0.0-rc.2.22431.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
arm64 [C:\Program Files\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\arm64\InstallLocation]
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
C:\Users\Agmas\source\repos\JsonSourceGenIEnumerableRepro\global.json

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

</details>

[martincostello]

I think the root cause will be the same as #43236, which is due to changes to S.T.J itself.

[JakeYallop]

I don't think that this is the same underlying issue.

In #43236, the source generated context has no direct reference to the underlying ProblemDetails type, whereas is this case the context has all the information it needs (we have explicitly opted in with a JsonSerializable attribute on the context), as demonstrated by the fact that serialization via JsonSerializer.Serialize using the source-generated JsonTypeInfo is successful.

There should be no need for a custom type resolver in this case.

Interestingly, using a List<MyClass> instead of an IEnumerable<MyClass> appears to work without an issue.

[JakeYallop]

Another data point - a nested IEnumerable property serializes successfully without any errors.

[JsonSerializable(typeof(NestedEnumerable))]
public partial class JsonContext : JsonSerializerContext { }

//...
public class NestedEnumerable
{
    public IEnumerable<MyClass> NestedIEnumerable { get; set; } = null!;
}

//...
[HttpGet("nested")]
public IActionResult Get2()
{
    var l2 = new NestedEnumerable()
    {
        NestedIEnumerable = new List<MyClass>() { new(), new(), new() }
    };
    return Ok(l2);
}

[davidfowl]

The issue is [JsonSerializable(typeof(IEnumerable<MyClass>))] vs [JsonSerializable(typeof(List<MyClass>))].

This explicit call:

var output = JsonSerializer.Serialize(l, JsonContext.Default.IEnumerableMyClass);

Is not the moral equivalent of what MVC is doing. The generic is inferred as IEnumerable<MyClass> so it works:

The equivalent would be this:

var options = new JsonSerializerOptions(JsonSerializerDefaults.Web);
options.AddContext<JsonContext>();
await System.Text.Json.JsonSerializer.SerializeAsync(new MemoryStream(), l, l.GetType(), options);

Which also throws. MVC passes the concrete type (which is List<MyClass>) to serialize.

TL;DR change your code to this:

[JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
[JsonSerializable(typeof(List<MyClass>))]
public partial class JsonContext : JsonSerializerContext
{

}

And it should work.

[JakeYallop]

@davidfowl, in this simplified example I know exactly what the underlying type is, and could use List<T> - what about if I had a HashSet<T>, Collection<T>, or some other collection returned from a method that returned IEnumerable?

The important thing to realise is that this surfaces as a regression and will break existing code that updates to .NET 7.0. I also think its totally reasonable that we might want to use the abstraction IEnumerable<T>, rather than a concrete type (which we might not know ahead of time).

Good point about async serialization - I was thinking about that yesterday, that, of course, ASP .NET Core is going to be using the async serialization code path.

I'll open another issue in the .NET runtime repo to surface this change in behaviour, thanks for your help!

[davidfowl]

Why are you using the source generator at all?

[JakeYallop]

Just to get experience using it in a personal project - I don't really need the additional perf in this case. I think this behavioural change might be most likely to surface in performance focused libraries that would want to be a) high performance and b) trim safe, that use async serialization behind the scenes.