Minimal API F# RouteValue is Null

[rfvgyhn]

Mapping a route with a route value in F# fails to bind the value when using a named function instead of a anonymous function. I'm not sure if this is an ASP.NET or F# issue (or maybe just a consequence of how F# works with the delegate param).

dotnet new web -lang F#

open System
open Microsoft.AspNetCore.Builder
open Microsoft.Extensions.Hosting

[<EntryPoint>]
let main args =
    let builder = WebApplication.CreateBuilder(args)
    let app = builder.Build()

    let funcB v = v
    let funcC = fun v -> v 

    app.MapGet("/a/{v}", Func<string, _> (fun v -> v)) |> ignore
    app.MapGet("/b/{v}", Func<string, _> funcB) |> ignore
    app.MapGet("/c/{v}", Func<string, _> funcC) |> ignore

    app.Run()

    0 // Exit code

dotnet run

GET /a/test

test

GET /b/test

System.ArgumentNullException: Value cannot be null. (Parameter 'text')
   at Microsoft.AspNetCore.Http.HttpResponseWritingExtensions.WriteAsync(HttpResponse response, String text, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Http.RequestDelegateFactory.ExecuteWriteStringResponseAsync(HttpContext httpContext, String text)
   at lambda_method2(Closure , Object , HttpContext )
   at Microsoft.AspNetCore.Http.RequestDelegateFactory.<>c__DisplayClass36_0.<Create>b__0(HttpContext httpContext)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

GET /c/test

System.ArgumentNullException: Value cannot be null. (Parameter 'text')
   at Microsoft.AspNetCore.Http.HttpResponseWritingExtensions.WriteAsync(HttpResponse response, String text, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Http.RequestDelegateFactory.ExecuteWriteStringResponseAsync(HttpContext httpContext, String text)
   at lambda_method3(Closure , Object , HttpContext )
   at Microsoft.AspNetCore.Http.RequestDelegateFactory.<>c__DisplayClass36_0.<Create>b__0(HttpContext httpContext)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

$ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.100
 Commit:    9e8b04bbff

Runtime Environment:
 OS Name:     arch
 OS Version:
 OS Platform: Linux
 RID:         arch-x64
 Base Path:   /usr/share/dotnet/sdk/6.0.100/

Host (useful for support):
  Version: 6.0.0
  Commit:  4822e3c3aa

.NET SDKs installed:
  6.0.100 [/usr/share/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.0 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.0 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

[davidfowl]

This is because the name of the parameter is lost through in the F# function conversions (as you've pointed out).

cc @dsyme

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[mchylek]

As a workaround you can also use wrapper with AsParametersAttribute

public static class Endpoint
{
    public static Delegate Of<TParam, TResult>(FSharpFunc<TParam, TResult> requestDelegate)
    {
        return (HttpContext context, [AsParametersAttribute] TParam parameters) => requestDelegate.Invoke(parameters);
    }
}

And then from F#

app.MapGet("/{name}", Endpoint.Of (fun (req : {| Name : string |}) -> $"Hello {req.Name}!"))

I've made simplistic library for this https://github.com/mchylek/Endpoints.FSharp.Interop

[lucasteles]

Also https://github.com/dotnet/aspnetcore/issues/46551#issuecomment-1810457875