.Net 9 RC1 - Cannot set reponse headers when returning IAsyncEnumerable in controller

[gabynevada]

Is there an existing issue for this?

• [X] I have searched the existing issues

Describe the bug

Had similar issues on .NET 6 release, adding just in case it's related:

• https://github.com/dotnet/aspnetcore/issues/35364
• https://github.com/dotnet/aspnetcore/issues/36977 (Was fixed after this one)

In .Net 9 RC1 when trying to set a header before yielding the first value on an IAsyncEnumerable, I get the following error:

      System.InvalidOperationException: Headers are read-only, response has already started.
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpHeaders.ThrowHeadersReadOnlyException()
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpHeaders.System.Collections.Generic.IDictionary<System.String,Microsoft.Extensions.Primitives.StringValues>.Add(String key, StringValues value)

Implementation of Controller looks like this:

    [HttpGet("set-header-aysnc-enumerable-error")]
    public async IAsyncEnumerable<WeatherForecastData> GetLocal([EnumeratorCancellation] CancellationToken cancellationToken)
    {
        var results = await dataService.GetData();
        // Adding a response header before yielding first result errors out
        Response.Headers.Add("Test", results.Name);
        await foreach (var result in results.Data.WithCancellation(cancellationToken))
        {
            yield return result;
        }
    }

Expected Behavior

The header to be set without errors as is possible in .NET 8

Usecase

We use this mostly to set Pagination headers for paginated lists

Steps To Reproduce

Use this webapi minimal reproduction

https://github.com/gabynevada/.net6-iasync-enumerable-set-header-error

Call endpoint returning IAsyncEnumerable

GET http://localhost:5000/api/data/set-header-aysnc-enumerable-error

Can replicate both on RC1 and the daily build for RC2

Exceptions (if any)

Connection id "0HN6MDAM4CBVH", Request id "0HN6MDAM4CBVH:00000001": An unhandled exception was thrown by the application.
      System.InvalidOperationException: Headers are read-only, response has already started.
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpHeaders.ThrowHeadersReadOnlyException()
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpHeaders.System.Collections.Generic.IDictionary<System.String,Microsoft.Extensions.Primitives.StringValues>.Add(String key, StringValues value)
         at SetResponseHeaders.Controllers.WeatherForecastController.GetLocal(CancellationToken cancellationToken)+MoveNext() in /Users/elvisnievesmiranda/RiderProjects/.net6-iasync-enumerable-set-header-error/Controllers/WeatherForecastController.cs:line 18
         at SetResponseHeaders.Controllers.WeatherForecastController.GetLocal(CancellationToken cancellationToken)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()
         at System.Text.Json.Serialization.Converters.IAsyncEnumerableOfTConverter`2.OnWriteResume(Utf8JsonWriter writer, TAsyncEnumerable value, JsonSerializerOptions options, WriteStack& state)
         at System.Text.Json.Serialization.JsonCollectionConverter`2.OnTryWrite(Utf8JsonWriter writer, TCollection value, JsonSerializerOptions options, WriteStack& state)
         at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
         at System.Text.Json.Serialization.JsonConverter`1.WriteCore(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
         at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.SerializeAsync(PipeWriter pipeWriter, T rootValue, Int32 flushThreshold, CancellationToken cancellationToken, Object rootValueBoxed)
         at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.SerializeAsync(PipeWriter pipeWriter, T rootValue, Int32 flushThreshold, CancellationToken cancellationToken, Object rootValueBoxed)
         at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.SerializeAsync(PipeWriter pipeWriter, T rootValue, Int32 flushThreshold, CancellationToken cancellationToken, Object rootValueBoxed)
         at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.SerializeAsync(PipeWriter pipeWriter, T rootValue, Int32 flushThreshold, CancellationToken cancellationToken, Object rootValueBoxed)
         at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeResultAsync>g__Logged|22_0(ResourceInvoker invoker, IActionResult result)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResultFilterAsync>g__Awaited|30_0[TFilter,TFilterAsync](ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters()
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|20_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Logged|17_1(ResourceInvoker invoker)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Logged|17_1(ResourceInvoker invoker)
         at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
         at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)

.NET Version

9.0.100-rc.2.24466.1

Anything else?

IDE: JetBrains Rider 2024.2.4 on Mac M1 Arm 64

dotnet --info:

dotnet --info
.NET SDK:
 Version:           9.0.100-rc.2.24466.1
 Commit:            c4104e5646
 Workload version:  9.0.100-manifests.792c81b2
 MSBuild version:   17.12.0-preview-24463-04+8500d97af

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  14.6
 OS Platform: Darwin
 RID:         osx-arm64
 Base Path:   /usr/local/share/dotnet/sdk/9.0.100-rc.2.24466.1/

.NET workloads installed:
Configured to use loose manifests when installing new manifests.
There are no installed workloads to display.

Host:
  Version:      9.0.0-rc.2.24462.10
  Architecture: arm64
  Commit:       static

[gabynevada]

Update, with minimal api it works.

The problem appears to be in Controllers:

app.MapGet(
    "/minimal-api",
    async (
        [FromServices] IDataService dataService,
        HttpContext httpContext,
        CancellationToken cancellationToken
    ) =>
    {
        var results = await dataService.GetData();

        httpContext.Response.Headers.Append("Test", results.Name);
        return results.Data;
    }
);

Sadly since it's a legacy app it will take us a while to migrate completely to minimal apis.

[BrennanConroy]

https://github.com/dotnet/aspnetcore/pull/57924 will fix this