HeaderDictionary can't add existing key

[tenor]

The header dictionary won't allow a header that already exists in the dictionary to be added.

            Response.Headers.Add("Hello", new string[] { "One" });
            Response.Headers.Add("Hello", new string[] { "Two" });

The code above will throw an exception on the second addition.
This was permissible in Web API 2.

        //Web API 2 Example
        // GET api/values
        public IHttpActionResult Get()
        {
            var response = Request.CreateResponse(HttpStatusCode.OK);
            response.Headers.Add("Hello", new string[] { "One" });
            response.Headers.Add("Hello", new string[] { "Two", "Three" });
            return ResponseMessage(response);
        }

In this scenario, Web API 2 adds the header values to the underlying values collection. I think this is the right behavior because that was the developer's intention even though they broke the contract.

[davidfowl]

Why not just call

Response.Headers.Append("Hello", "One");
Response.Headers.Append("Hello", "Two");

[Tratcher]

FYI: That's the required contract for IDictionary. WebApi didn't implement IDictionary.

Yes, you should use the Append extension for this behavior.

[tenor]

I don't feel strongly either way about this. Append is an extension that is not visible unless I'm already aware of it.
I'm just pointing out that one way breaks a working pattern in older apps, and the other way properly adheres to the IDictionary contract.

[davidfowl]

This is one of those things that I think needs to change in application code. I think Append could be closer to the actual type since it's a fundamental operation (or is it?).

[davidfowl]

I'm moving those extension methods into Http.Abstractions.