Minimal API Group with OpenAPI and Return Type of Result<TResult> creates incorrect OpenAPI document.

[MarkTallentire]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I have a simple API that uses the new OpenAPI document generation and Minimal APIs that utilize groups functionality.

When using the following code, i expect OpenAPI to generate a document with the available return types of Created and BadRequest.

However I get Ok, Created and BadRequest and it also produces some weird return model types

I also see the following exception in my console.

System.ArgumentException: An item with the same key has already been added. Key: OpenApiSchemaKey { Type = Microsoft.AspNetCore.Http.HttpResults.Results2[Microsoft.AspNetCore.Http.HttpResults.Created1[Api.Features.Accounts.DTOs.CreateAccountResponse],Microsoft.AspNetCore.Http.HttpResults.BadRequest1[FluentValidation.Results.ValidationResult]], ParameterInfo = }
`

using Api.Features.Accounts.DTOs;
using Api.Features.Accounts.Services;
using Api.Features.Accounts.Validators;
using Domain;
using FluentValidation.Results;
using Microsoft.AspNetCore.Http.HttpResults;

namespace Api.Features.Accounts.Endpoints
{
    public static class AccountsEndpoints
    {
        public static RouteGroupBuilder MapAccountsEndpoints(this RouteGroupBuilder group)
        {
            group.MapPost("/", CreateAccount)
                .AllowAnonymous();

            return group;
        }

        public static async Task<Results<Created<CreateAccountResponse>, BadRequest<ValidationResult>>> CreateAccount(CreateAccountRequest accountRequest, AccountService accountService)
        {
            var validator = new CreateAccountRequestValidator();
            var validationResult = await validator.ValidateAsync(accountRequest);

            if (!validationResult.IsValid)
            {
                return TypedResults.BadRequest<ValidationResult>(validationResult);
            }
            
            var account = await accountService.CreateAccount(accountRequest.Username);

            if (account == null)
            {
                validationResult.Errors.Add(new ValidationFailure(nameof(accountRequest.Username), "Username already exists"));
                return TypedResults.BadRequest<ValidationResult>(validationResult);
            }

            return TypedResults.Created<CreateAccountResponse>(new Uri("https://google.com"), new CreateAccountResponse(account.ApiKey));
        }
    }
}

builder.Services.AddOpenApi();
if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
    app.MapScalarApiReference();
}
app.MapGroup("accounts")
    .MapAccountsEndpoints();

Produces:

Expected Behavior

To show only Created(201) and BadRequest(400) as available return types and only ValidationResult and CreateAccountResponse as return models.

Steps To Reproduce

Use code above in a new .net9 project and run

Exceptions (if any)

System.ArgumentException: An item with the same key has already been added. Key: OpenApiSchemaKey { Type = Microsoft.AspNetCore.Http.HttpResults.Results2[Microsoft.AspNetCore.Http.HttpResults.Created1[Api.Features.Accounts.DTOs.CreateAccountResponse],Microsoft.AspNetCore.Http.HttpResults.BadRequest1[FluentValidation.Results.ValidationResult]], ParameterInfo = }
`

.NET Version

9.0 preview

Anything else?

I have also attempted to use .Produces() to explicitly define the return types but have the same issue

[martincostello]

Which preview of .NET 9 are you using? If not release candidate 1, try upgrading to that - there's a bunch of bug fixes since preview 7.

[MarkTallentire]

@martincostello thanks for the quick response. I updated to 9.0.100-rc.1.24452.12 and it does indeed fix the documentation

Do however still see an exception, although slightly different to the original one.

 Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
      System.ArgumentException: An item with the same key has already been added. Key: OpenApiSchemaKey { Type = Api.Features.Accounts.DTOs.CreateAccountResponse, ParameterInfo =  }
         at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
         at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
         at Microsoft.AspNetCore.OpenApi.OpenApiSchemaStore.GetOrAdd(OpenApiSchemaKey key, Func`2 valueFactory)
         at Microsoft.AspNetCore.OpenApi.OpenApiSchemaService.GetOrCreateSchemaAsync(Type type, IServiceProvider scopedServiceProvider, IOpenApiSchemaTransformer[] schemaTransformers, ApiParameterDescription parameterDescription, Boolean captureSchemaByRef, CancellationToken cancellationToken)
         at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetResponseAsync(ApiDescription apiDescription, Int32 statusCode, ApiResponseType apiResponseType, IServiceProvider scopedServiceProvider, IOpenApiSchemaTransformer[] schemaTransformers, CancellationToken cancellationToken)
         at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetResponsesAsync(ApiDescription description, IServiceProvider scopedServiceProvider, IOpenApiSchemaTransformer[] schemaTransformers, CancellationToken cancellationToken)
         at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetOperationAsync(ApiDescription description, HashSet`1 capturedTags, IServiceProvider scopedServiceProvider, IOpenApiSchemaTransformer[] schemaTransformers, CancellationToken cancellationToken)
         at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetOperationsAsync(IGrouping`2 descriptions, HashSet`1 capturedTags, IServiceProvider scopedServiceProvider, IOpenApiOperationTransformer[] operationTransformers, IOpenApiSchemaTransformer[] schemaTransformers, CancellationToken cancellationToken)
         at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetOpenApiPathsAsync(HashSet`1 capturedTags, IServiceProvider scopedServiceProvider, IOpenApiOperationTransformer[] operationTransformers, IOpenApiSchemaTransformer[] schemaTransformers, CancellationToken cancellationToken)
         at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetOpenApiDocumentAsync(IServiceProvider scopedServiceProvider, CancellationToken cancellationToken)
         at Microsoft.AspNetCore.Builder.OpenApiEndpointRouteBuilderExtensions.<>c__DisplayClass0_0.<<MapOpenApi>b__0>d.MoveNext()
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Http.Generated.<GeneratedRouteBuilderExtensions_g>F56B68D2B55B5B7B373BA2E4796D897848BC0F04A969B1AF6260183E8B9E0BAF2__GeneratedRouteBuilderExtensionsCore.<>c__DisplayClass2_0.<<MapGet0>g__RequestHandler|5>d.MoveNext()
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

[captainsafia]

@MarkTallentire Thanks for filling this issue! I'm glad @martincostello's guidance about updating to RC1 solved your first issue.

As for the second one, does the issue only happen when you access the Scalar UI? Does something similar happen when you directly navigate to the /openapi/v1.json (or whatever URL you've configured) for your endpoint on a freshly spun up server?

[MarkTallentire]

@captainsafia I can confirm this only happens when I access the scalar UI, possibly a bug on their side instead of in .net?

[martincostello]

It might be that your second issue was fixed by #57852.

[captainsafia]

It might be that your second issue was fixed by #57852.

I don't think anything in that PR would've resolved this particularly issue but I suppose it is worth a try. You can try it out by using package version 9.0.0-rc.2.24466.8 from the nightly dotnet9 package feed. You can find directions on how to access the feed in this section of the docs.

[xC0dex]

Hey @MarkTallentire,
I reviewed the source code of the Scalar.AspNetCore package. However, I was unable to find any code paths that could cause this error. The library doesn't manipulate the generated document. Only the openapi/v1.json path is referenced.

[MarkTallentire]

@xC0dex Thanks for checking. @captainsafia @martincostello I updated to latest version starting with 9 available in the nightly nuget provided above but unfortunately still see the error

I've created a repo that reproduces this issue in the hope it helps diagnose. I get the error 8/10 times when running directly from a terminal using dotnet run and loading scalar/v1 in my browser.

The only changes made from the default webapi template built using dotnet new webapi are:
Install Scalar.AspNetCore and add app.MapScalarApiReference() to Program.cs
Upgrade Microsoft.AspNetCore.OpenApi to 9.0.0-rtm.24466.12
Update launchsettings.json to directly launch scalar/v1 instead of weatherforecast

Github Repo

Thanks again