How to set custom headers when using IHttpActionResult?

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.

You can create a HttpResponseMessage, add headers as needed and then create ResponseMessageResult from it:

HttpResponseMessage response =new HttpResponseMessage(HttpStatusCode.OK);
response.Headers.Add("MyHeader", "MyHeaderValue");
return ResponseMessage(response);