如何从Web API 2强制使用Content-Type MediaTypeFormatter

Question

I have an issue with my MediaTypeFormatter. When I make a request with the Accept header set to "application/vnd.siren+json", it correctly sets the response to have Content Type header set to "application/vnd.siren+json".

What I am trying to do is even when I do not state explicitly that I want "application/vnd.siren+json", I would like to set the response content type to be "application/vnd.siren+json".

For example, a bog-standard call will have this Accept header set:

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8

When I do a GET using that Accept header, my response type will be application/xml and not application/vnd.siren+json.

WebApiConfig.cs has been configured as:

SirenMediaTypeFormatter sirenMediaTypeFormatter = new SirenMediaTypeFormatter();
sirenMediaTypeFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/xml"));
sirenMediaTypeFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/vnd.siren+json"));
config.Formatters.Insert(0, sirenMediaTypeFormatter);

I have set up my MediaTypeFormatter as:

public class SirenMediaTypeFormatter : JsonMediaTypeFormatter 
{
    private static Type _supportedType = typeof(Entity);
    private const string _mediaType = "application/vnd.siren+json";

    public SirenMediaTypeFormatter()
    {
        SupportedMediaTypes.Add(new MediaTypeHeaderValue(_mediaType));
    }

    public override bool CanReadType(Type type)
    {
        return type == _supportedType;
    }

    public override bool CanWriteType(Type type)
    {
        bool blnRetval = (typeof(Entity).IsAssignableFrom(type));
        return blnRetval;
    }


    public override Task WriteToStreamAsync(Type type, object value,
   Stream stream, System.Net.Http.HttpContent content, System.Net.TransportContext transportContext)
    {
        return Task.Factory.StartNew(() =>
        {
            if (typeof(Entity).IsAssignableFrom(type))
            {
                content.Headers.ContentType = new MediaTypeHeaderValue(_mediaType);

                var objectToSerialize = BuildSirenDocument(value, stream, content.Headers.ContentType.MediaType);


                var jsonSerializerSettings = new JsonSerializerSettings
                {
                    ContractResolver = new CamelCasePropertyNamesContractResolver()
                };

                string jsonResult = Newtonsoft.Json.JsonConvert.SerializeObject(objectToSerialize, jsonSerializerSettings);

                StreamWriter writer = new StreamWriter(stream);
                writer.Write(jsonResult);
                writer.Flush();

            }
        });
    }

I have tried to update the value of the Context using content.Headers.ContentType = new MediaTypeHeaderValue(_mediaType); however 1). it does not work and the content type remains set to application/xml and 2). I worry about playing around with Context in a WriteAsynch method like this.

How do I go about forcing my Response header type (without setting it explicitly in the controller).

Answer 1

It's too late to write the headers by the time WriteToStreamAsync is called as you allude to in your question. Instead you need to override SetDefaultContentHeaders .

From the documentation, this:

Sets the default headers for content that will be formatted using this formatter.

To change the Content Type you can pass through your own MediaTypeHeaderValue to the base method:

public override void SetDefaultContentHeaders(Type type, HttpContentHeaders headers, MediaTypeHeaderValue mediaType)
{
    base.SetDefaultContentHeaders(type, headers, new MediaTypeHeaderValue(_mediaType));
}