How to load an OpenAPI description document with an external schema

[alfredmyers]

Given the OpenAPI description document at https://www.bcb.gov.br/htms/dasfn/catalogo/1.0.10/openapi.json referencing the external schema file at https://www.bcb.gov.br/htms/dasfn/catalogo/1.0.10/schema.json, the following code

using Microsoft.OpenApi;
using Microsoft.OpenApi.Reader;

namespace Dasfn
{
    internal class Program
    {
        static async Task Main(string[] args)
        {
            Uri baseUrl = new("https://www.bcb.gov.br/htms/dasfn/catalogo/1.0.10/");

            OpenApiReaderSettings settings = new()
            {
                BaseUrl = baseUrl,
                LoadExternalRefs = true,
            };

            ReadResult readResult = await OpenApiDocument.LoadAsync(new Uri(baseUrl, "openapi.json").AbsoluteUri, settings);

            if (readResult.Diagnostic?.Errors is IList<OpenApiError> errors)
            {
                foreach (OpenApiError error in errors)
                { 
                    Console.WriteLine(error); 
                }
            }

            OpenApiDocument document = readResult.Document!;

            OpenApiSchemaReference schema = (OpenApiSchemaReference) document
                .Paths["/catalogo"]
                .Operations![HttpMethod.Get]
                .Responses!["200"]
                .Content!["application/json"].Schema!
                ;

            Console.WriteLine($"Schema unresolved reference: {schema.UnresolvedReference}");
        }
    }
}

... reports failure to load schema.json apparently due to it not having a Version field. Furthermore, the schema is marked as an unsolved reference:

[File: schema.json] Version node not found.
Schema unresolved reference: True

It is my understanding that Version is only required on the OpenAPI description itself and not on referenced schemas.

I have no control over the definition of this OpenAPI description document so changing it is not an option.

Is there a proper way or a workaround for loading the external schema?

[alfredmyers]

I've got it working by patching the OpenAPI description document with a fragment containing the schema

using Microsoft.OpenApi;
using Microsoft.OpenApi.Reader;
using System.Text.Json.Nodes;

namespace Dasfn
{
    internal class Program
    {
        static async Task Main(string[] args)
        {
            Uri baseUrl = new("https://www.bcb.gov.br/htms/dasfn/catalogo/1.0.10/");

            HttpClient httpClient = new()
            {
                BaseAddress = baseUrl,
            };

            OpenApiReaderSettings settings = new()
            {
                BaseUrl = baseUrl,
                HttpClient = httpClient,
                LoadExternalRefs = true,
            };

            (OpenApiDocument? document, OpenApiDiagnostic? diagnostic) = await OpenApiDocument.LoadAsync(new Uri(baseUrl, "openapi.json").AbsoluteUri, settings);

            if (diagnostic?.Errors is IList<OpenApiError> errors)
            {
                foreach (OpenApiError error in errors)
                { 
                    Console.WriteLine(error); 
                }
            }

            OpenApiMediaType mediaType = document!
                .Paths["/catalogo"]
                .Operations![HttpMethod.Get]
                .Responses!["200"]
                .Content!["application/json"]
                ;

            if (mediaType.Schema is OpenApiSchemaReference schemaReference && schemaReference.UnresolvedReference)
            {
                using MemoryStream stream = new(await httpClient.GetByteArrayAsync(schemaReference.Reference.ExternalResource, CancellationToken.None));

                mediaType.Schema = settings.Readers["json"]
                    .ReadFragment<OpenApiSchema>(stream, diagnostic!.SpecificationVersion, document, out OpenApiDiagnostic schemaDiagnostic);
            }
        }
    }
}

There's probably a better/proper way to do this, but at least for now this will suffice.