Support custom task types in Minimal API endpoints

[ericmutta]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe the problem.

I am trying to map a minimal API endpoint as follows:

  ArgApp.MapPost("/api/create-account", CreateAccount.HandleHttpAsync);

...the endpoint itself is defined with this signature:

 public static async AwaitOnceTask<Response> HandleHttpAsync(HttpRequest ArgRequest, IServiceProvider ArgServices) { }

When I try to run my app, I get the following exception message and stack trace:

System.NotSupportedException: Unsupported return type: AwaitOnceTask<CreateAccount.Response>
         at Microsoft.AspNetCore.Http.RequestDelegateFactory.PopulateBuiltInResponseTypeMetadata(Type returnType, EndpointBuilder builder)
         at Microsoft.AspNetCore.Http.RequestDelegateFactory.CreateArgumentsAndInferMetadata(MethodInfo methodInfo, RequestDelegateFactoryContext factoryContext)
         at Microsoft.AspNetCore.Http.RequestDelegateFactory.InferMetadata(MethodInfo methodInfo, RequestDelegateFactoryOptions options)
         at Microsoft.AspNetCore.Routing.RouteEndpointDataSource.CreateRouteEndpointBuilder(RouteEntry entry, RoutePattern groupPrefix, IReadOnlyList`1 groupConventions, IReadOnlyList`1 groupFinallyConventions)
         at Microsoft.AspNetCore.Routing.RouteEndpointDataSource.get_Endpoints()

The problem seems to be the return type AwaitOnceTask<CreateAccount.Response> which uses a custom task type I have created called AwaitOnceTask<T>.

Describe the solution you'd like

The docs currently say that:

Minimal endpoints support the following types of return values:

1. string - This includes Task<string> and ValueTask<string>.
2. T (Any other type) - This includes Task<T> and ValueTask<T>.
3. IResult based - This includes Task<IResult> and ValueTask<IResult>.

...this means support for async endpoints is already included but seems to be hardcoded to just Task<T> or ValueTask<T> . It would be great if the logic in Microsoft.AspNetCore.Http.RequestDelegateFactory was written to support any task type that can be defined in C#, not just the native Task<T> or ValueTask<T>.

Additional context

No response

[ericmutta]

CC: @danmoseley ...I just started using Minimal APIs which have come a very long way in .NET 8 (which also happens to be the first .NET that contains code I had the pleasure of contributing 😃)...this issue may be one of the remaining edge cases to address in coming releases of .NET 👍

[captainsafia]

@ericmutta Thanks for reporting this!

It's a compelling feature but there's definitely a hefty testing/design requirement for this one. I'll stick this in the backlog for now to track interest in it. When we do approach this issue, we should think about:

• How this intersects with endpoints filters on an application
• How this intersects with ApiExplorer/OpenAPI tooling
• How to make sure this is compatible with the static code generation story for minimal APIs via Request Delegate Generator

[ericmutta]

Many thanks @captainsafia for following up 👍

[Youssef1313]

This sounds like a duplicate of #40554.