Generate YAML using Microsoft.AspNetCore.OpenApi

OpenApiYamlEndpoint.cs

using System.Diagnostics.CodeAnalysis;
using Microsoft.Extensions.Options;
using Microsoft.OpenApi.Extensions;
using Microsoft.OpenApi.Models;
using Microsoft.OpenApi.Writers;

namespace Microsoft.AspNetCore.OpenApi;

public static class OpenApiYamlEndpoint
{
    public static IEndpointConventionBuilder MapOpenApiYaml(
        this IEndpointRouteBuilder endpoints,
        [StringSyntax("Route")] string pattern = "/openapi/{documentName}.yml")
    {
        var type = Type.GetType("Microsoft.AspNetCore.OpenApi.OpenApiDocumentService, Microsoft.AspNetCore.OpenApi");
        var methodInfo = type!.GetMethod("GetOpenApiDocumentAsync")!;

        var options = endpoints.ServiceProvider.GetRequiredService<IOptionsMonitor<OpenApiOptions>>();
        return endpoints.MapGet(pattern, async (HttpContext context, string documentName = "v1") =>
        {
            object? documentService = context.RequestServices.GetRequiredKeyedService(type!, documentName);
            if (documentService is null)
            {
                context.Response.StatusCode = StatusCodes.Status404NotFound;
                context.Response.ContentType = "text/plain;charset=utf-8";
                await context.Response.WriteAsync($"No OpenAPI document with the name '{documentName}' was found.");
            }
            else
            {
                var document = await GetOpenApiDocumentAsync(documentService, context.RequestAborted);
                var documentOptions = options.Get(documentName);

                using var memory = new MemoryStream();
                using var streamWriter = new StreamWriter(memory);
                var yamlWriter = new OpenApiYamlWriter(streamWriter);

                document.Serialize(yamlWriter, documentOptions.OpenApiVersion);
                context.Response.ContentType = "application/yaml";
                await context.Response.BodyWriter.WriteAsync(memory.ToArray(), context.RequestAborted);
                await context.Response.BodyWriter.FlushAsync(context.RequestAborted);
            }
        }).ExcludeFromDescription();

        async Task<OpenApiDocument> GetOpenApiDocumentAsync(object documentService, CancellationToken cancellationToken)
        {
            var result = (Task<OpenApiDocument>)methodInfo.Invoke(documentService, [cancellationToken])!;
            return await result;
        }
    }
}