Adding multiple allow headers is only adding the last header in http response - asp.net-core

Question

I am attempting to add multiple Allow Headers to the HttpResponse in aspnetcore v1.1.2. When I run the code below, the headers are added to the IHeaderDictionary on the HttpResponse, however only the last header (in this case "POST") in the collection is actually added to the http response when serialized. Has anyone else experienced this, or am I doing something wrong??
Here is the code I am using.
public override async Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next)
{
context.HttpContext.Response.Headers.Add("Allow", new StringValues(new [] {"GET", "POST"}));
await next();
}
I am doing this within a ResultFilterAttribute.
Many Thanks...

Answer 1

This should fix the issue:
context.HttpContext.Response.Headers.Add("Allow", "GET, POST");

Answer 2

Your code is fine, the only place that you may need to change is your response parsing logic. Double check your response - it contains 2 Allow headers, not one header with 2 values:
Allow: GET
Allow: POST
Simple example:
Lets say you have next controller action:
[Route("api/[controller]")]
public class ValuesController : Controller
{
// GET api/values
[HttpGet]
[AddHeader]
public IEnumerable<string> Get()
{
return new string[] { "value1", "value2" };
}
}
Get request using curl:
curl -X GET http://localhost:5000/api/values -i
Response:
HTTP/1.1 200 OK
Date: Fri, 23 Jun 2017 22:23:24 GMT
Content-Type: application/json; charset=utf-8
Server: Kestrel
Transfer-Encoding: chunked
Allow: GET
Allow: POST
["value1","value2"]