Adding custom headers to Azure Functions response

Question

I'm working on an Azure Function App (v2), which has a number of different functions inside the same Function App.

It is going to be deployed in multiple Azure regions, and I would like to add a custom header, that indicates which region served the request.

I know that I can let each function return HttpResponseMessage which can contain custom headers. Instead of duplicating that code to each function, I would like to make it centralized in the entire Function App project. Another drawback of returning HttpResponseMessage is that I would like to use IActionResult, so I can use JsonResult or OkObjectResult and similar return types.

With an ASP.NET MVC project, I could add a middleware which added the header or add the headers using web.config. Neither seems to be possible in a Function App.

How can this be done without having to return HttpResponseMessage and adding the header inside each function's Run method?

Answer 1

I was able to do this by accessing the HttpResponse object via the request's HttpContext.

For example:

req.HttpContext.Response.Headers.Add("ETag", entity.ETag);
return new OkObjectResult(entity);

produces:

Answer 2

while researching the exact same problem, I came along this blog post. Basically, you derive a custom class fomr JsonResult, OKResult and so on and manipulate the ExecuteResult method and add your headers there.

public class ServiceUnavailableResult : StatusCodeResult {
    private readonly int _retryAfterHeaderValue;

    public ServiceUnavailableResult(int retryAfterHeaderValue) : base(500) {
        _retryAfterHeaderValue = retryAfterHeaderValue;
    }

    public override void ExecuteResult(ActionContext context) {
        base.ExecuteResult(context);
        
        context.HttpContext.Response.Headers.Add("Retry-After", _retryAfterHeaderValue.ToString());
    }
}

It does not fully help you, but if you only use like two or three classes when returning content, it might.