Execption thrown when generating OpenAPI document using F#

[josh-coproduct]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When using a very specific combination of types in F#, the OpenAPI document fails to generate and an exception is thrown. I believe it is a similar bug to #58213, but in this case the parent type is not referenced from a sub-type, which was the suspected cause in that case. Recreating the same code in C# failed to recreate the bug.

Specifically, all of these conditions have to be true to trigger the exception:

• Dog.favoriteFoods has to be an array, or at least not just a string
• Test1Result has to include fields of type Dog and Animal
• Both Animal and Test1Result have to be response types

open System
open System.Text.Json.Serialization
open Microsoft.AspNetCore.Builder
open Microsoft.Extensions.DependencyInjection
open Microsoft.Extensions.Hosting
open Microsoft.AspNetCore.Http

[<JsonDerivedType(typeof<Dog>)>]
[<JsonDerivedType(typeof<Cat>)>]
type Animal() =
    member val name = "" with get, set

and Cat() =
    inherit Animal()
    member val favoriteFood: string = "tuna" with get, set

and Dog() =
    inherit Animal()
    member val favoriteFoods: string[] = [||] with get, set

type Test1Result = { dog: Dog; animal: Animal }

[<EntryPoint>]
let main args =

    let builder = WebApplication.CreateBuilder(args)

    builder.Services.AddOpenApi() |> ignore

    let app = builder.Build()

    if app.Environment.IsDevelopment() then
        app.MapOpenApi() |> ignore

    app.MapGet("/test1", Func<string>(fun () -> "Hello World!"))
    |> _.Produces<Test1Result>(StatusCodes.Status200OK, null, [||])
    |> ignore

    app.MapGet("/test2", Func<string>(fun () -> "Hello World!"))
    |> _.Produces<Animal>(StatusCodes.Status200OK, null, [||])
    |> ignore

    app.Run()

    0

Expected Behavior

The OpenAPI document is successfully generated.

Steps To Reproduce

A minimalist project recreating the issue is here:

https://github.com/josh-coproduct/FSharp-OpenAPI-Bug-Sample

It includes the F# code the triggers the bug, and the C# code that does not.

Launching the F# project should open a browser window to http://localhost:5047/openapi/v1.json, which should trigger the bug and show the exception.

Exceptions (if any)

System.ArgumentException: An item with the same key has already been added. Key: AnimalDog
   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.OpenApiSchemaReferenceTransformer.TransformAsync(OpenApiDocument document, OpenApiDocumentTransformerContext context, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.ApplyTransformersAsync(OpenApiDocument document, IServiceProvider scopedServiceProvider, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetOpenApiDocumentAsync(IServiceProvider scopedServiceProvider, 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)

.NET Version

9.0.101

Anything else?

No response