[OpenApi] Preview 4: Cannot set Header on `OpenApiOperation`

[erwinkramer]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Headers are readonly on OpenApiOperation.

In a IOpenApiOperationTransformer my exception gets triggered for 200 responses (that i don't have control over, it's just simple 200 responses generated in a minimal API):

 private void AddHeadersToResponses(OpenApiOperation operation, OpenApiDocument document)
    {
        if (operation.Responses == null) return;

        foreach (var response in operation.Responses)
        {
            if(response.Value.Headers == null)
                throw new InvalidOperationException($"Headers is null and it's read only :(( @ {response.Key}"); // TODO

            response.Value.Headers["API-Version"] = new OpenApiHeaderReference("API-Version", document);
            response.Value.Headers["Access-Control-Allow-Origin"] = new OpenApiHeaderReference("Access-Control-Allow-Origin", document);
            response.Value.Headers["Access-Control-Expose-Headers"] = new OpenApiHeaderReference("GenericStringHeader", document);

            if (response.Key[0] is '2' or '4')
            {
                response.Value.Headers["X-Rate-Limit-Limit"] = new OpenApiHeaderReference("X-Rate-Limit-Limit", document);
            }
        }
    }

Expected Behavior

No response

Steps To Reproduce

Check the branch for this PR: erwinkramer/bank-api#6

Exceptions (if any)

No response

.NET Version

10.0.100-preview.4.25258.110

Anything else?

No response

[martincostello]

OpenApiOperation is defined by Microsoft.OpenApi and various changes were made to the object model in preview.17 compared to preview.11.

You can take a look at the upgrade guide here to get an idea of the changes and how to adapt your code to deal with them.

Some hints are that you need to use type assertions on the interface type, and then you should be able to set the headers. Also, collections now default to null so you need to initialize them too.

Here's a rough example of what you need to do:

foreach (var response in operation.Responses)
{
    if (response.Value is OpenApiResponse concrete)
    {
        concrete.Headers ??= [];
        concrete.Headers["API-Version"] = /* your existing code */
    }
}

Here's Swashbuckle.AspNetCore's changes to update for preview 4 as an example: domaindrivendev/Swashbuckle.AspNetCore#3386

[erwinkramer]

Thanks again, that works.