Add header parameter to endpoint OpenAPI definition

[ntark]

Problem:

I have IBindableFromHttpContext<CurrentLanguage> endpoint parameter that I am parsing from language header.
Problem is that it does not show up on OpenAPI/Swagger as a header parameter field.

I need "Accept-Language" header parameter to show up for endpoints that have CurrentLanguage Parameter

What I have tried

I believe for that CurrentLanguage should implement IEndpointParameterMetadataProvider and add corresponding metadata, but I have been unable to do so.

Just adding builder.Metadata.Add(new FromHeaderAttribute { Name = "Accept-Language" }); does nothing.

I also tried adding LanguageParameterMetadata(ParameterInfo parameter) : IParameterBindingMetadata metadata but thats no-go either, since that interface contains ParameterInfo parameter, which has a protected contructor and CustomAttributes are not editable.

Code Samples:

Minimal repro with problem

using Microsoft.AspNetCore.Mvc;
using System.Reflection;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddSwaggerGen();
var app = builder.Build();
app.UseSwagger();
app.UseSwaggerUI();
app.MapGet("/test", (CurrentLanguage language, [FromHeader] string testHeader) => language.Value.Code);
app.Run();

record CurrentLanguage(Language Value) : IBindableFromHttpContext<CurrentLanguage>
{
    public static ValueTask<CurrentLanguage?> BindAsync(HttpContext context, ParameterInfo parameter)
    {
        var langValue = context.Request.Headers.AcceptLanguage.FirstOrDefault() ?? "en";
        var language = new Language(langValue);
        return ValueTask.FromResult(new CurrentLanguage(language))!;
    }
}

record Language(string Code);

What fixes the problem but does not solve it:

Adding WithOpenApi

app.MapGet("/test", (CurrentLanguage language, [FromHeader] string testHeader) => language.Value.Code)
   .WithOpenApi(operation =>
   {
       operation.Parameters.Add(new OpenApiParameter
       {
           Name = "Accept-Language",
           In = ParameterLocation.Header,
           Required = false,
           Description = "Language preference (e.g. en-US, ka-GE)",
           Schema = new OpenApiSchema
           {
               Type = JsonSchemaType.String
           }
       });

       return operation;
   });

• cons:
  • Deprecated
  • Must be applied manually to each endpoint

Stealing ParameterInfo from other header parameter

record CurrentLanguage(Language Value) : IBindableFromHttpContext<CurrentLanguage>, IEndpointParameterMetadataProvider
{
    public static ValueTask<CurrentLanguage?> BindAsync(HttpContext context, ParameterInfo parameter)
    {
        var langValue = context.Request.Headers.AcceptLanguage.FirstOrDefault() ?? "en";
        var language = new Language(langValue);
        return ValueTask.FromResult(new CurrentLanguage(language))!;
    }

    public static void PopulateMetadata(ParameterInfo parameter, EndpointBuilder builder)
    {
        var p = (IParameterBindingMetadata)builder.Metadata[3];
        var m = new LanguageParameterBindingMetadata("Accept-Language", p.HasTryParse, p.HasBindAsync, p.ParameterInfo, p.IsOptional);
        builder.Metadata.Add(m);
    }
}

class LanguageParameterBindingMetadata(string name, bool hastryParse, bool hasBindAsync, ParameterInfo parameterInfo, bool isOptonal) 
    : IParameterBindingMetadata
{
    public string Name => name;
    public bool HasTryParse => hastryParse;
    public bool HasBindAsync => hasBindAsync;
    public ParameterInfo ParameterInfo => parameterInfo;
    public bool IsOptional => isOptonal;
}

• cons: Obviously a non-solution