How to set custom headers when using IHttpActionResult?

Question

In ASP.NET Web API 2, the IHttpActionResult offers a lot of value in simplifying controller code and I'm reluctant to stop using it, but I've hit a problem.

I need to set an ETag on an outgoing response, and I cannot find any property which gives me access to the response's headers. At the moment I'm using the Ok<T>(T content) helper method from the ApiController, which returns an OkNegotiatedContentResult<T> object. That doesn't seem to have anything exposed which would let me modify the headers though.

Am I missing something, or is there really no way to do this while using the stock IHttpActionResult types? I considered a message handler, but then I'd have to figure out how to pass the ETag out of the action (the ETags are generated differently for different actions, so it's not a matter of making a generic handler for all actions).

I'd like to avoid having to use the raw HttpResponseMessage, but at the moment that's looking difficult.

Answer 1

For your scenario, you would need to create a custom IHttpActionResult. Following is an example where I derive from OkNegotiatedContentResult<T> as it runs Content-Negotiation and sets the Ok status code.

public class CustomOkResult<T> : OkNegotiatedContentResult<T> {     public CustomOkResult(T content, ApiController controller)         : base(content, controller) { }      public CustomOkResult(T content, IContentNegotiator contentNegotiator, HttpRequestMessage request, IEnumerable<MediaTypeFormatter> formatters)          : base(content, contentNegotiator, request, formatters) { }      public string ETagValue { get; set; }      public override async Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)     {         HttpResponseMessage response = await base.ExecuteAsync(cancellationToken);          response.Headers.ETag = new EntityTagHeaderValue(this.ETagValue);          return response;     }         } 

Controller:

public class ValuesController : ApiController {     public IHttpActionResult Get()     {         return new CustomOkResult<string>(content: "Hello World!", controller: this)             {                     ETagValue = "You ETag value"             };     } } 

Note that you can also derive from NegotiatedContentResult<T>, in which case you would need to supply the StatusCode yourself. Hope this helps.

You can find the source code of OkNegotiatedContentResult<T> and NegotiatedContentResult<T>, which as you can imagine are simple actually.