ASP.NET Core web api action selection based on Accept header

Question

I want to return two different formatted responses for the same feature (a list of entities) based on the accept header of the request, it is for a "json" and a "html" request.

Does the asp.net core support select different actions for the same route based upon the Accept Header from the request?

ASP.NET Core already checks the Accept header to decide which output formatter to use. Check Content Negotiation and Configuring Formatters in the docs. What would an HTML formatter return though? What tags would it use? Should it use a specific template? — Panagiotis Kanavos
– Panagiotis Kanavos, Commented May 25, 2017 at 15:22
Also check Shawn Wildermuth's article on Content Negotiation — Panagiotis Kanavos
– Panagiotis Kanavos, Commented May 25, 2017 at 15:24
it is not just content negotiation, i want to return a View from one action (with all the viewengine / razor stuff / ModelSate) and a json serialization from the other one, even (im not sure by now) i will need different filters, or authorization attributes in each action. — animalito maquina
– animalito maquina, Commented May 25, 2017 at 15:34
Then you should be asking about MVC, not Web API. Content negotiation works there too. The only problem is that you need to check the Accept header and use View() when html is requested, Ok() otherwise. — Panagiotis Kanavos
– Panagiotis Kanavos, Commented May 25, 2017 at 16:10
The nice thing is that you don't need to perform any extra configuration or add any services, unless you want to add more formatters, eg for XML or word. Using Ok() ensures that the formatter that matches Accept will be used — Panagiotis Kanavos
– Panagiotis Kanavos, Commented May 25, 2017 at 16:19

animalito maquina · Accepted Answer · 2018-02-22 10:16:16Z

I dived into the .net core source code and looked for other attributes that do some similar behaviour such as Microsoft.AspNetCore.Mvc.HttpGet or Microsoft.AspNetCore.Mvc.ProducesAttribute. Both attributes implements an Microsoft.AspNetCore.Mvc.ActionConstraints.IActionConstraint interface wich is used by aspnetcore.mvc to control the selection of actions inside a controller.

So i implemented a simplified ProducesAttribute (a "tribute") to check for the accept header.

    /// <summary>
    /// A filter that specifies the supported response content types. The request accept header is used to determine if it is a valid action
    /// </summary>
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
    public class AcceptHeaderAttribute : Attribute, IActionConstraint
    {

        public AcceptHeaderAttribute(string contentType, params string[] otherContentTypes)
        {            
            if (contentType == null)
                throw new ArgumentNullException(nameof(contentType));

            // We want to ensure that the given provided content types are valid values, so
            // we validate them using the semantics of MediaTypeHeaderValue.
            MediaTypeHeaderValue.Parse(contentType);

            for (var i = 0; i < otherContentTypes.Length; i++)
            {
                MediaTypeHeaderValue.Parse(otherContentTypes[i]);
            }

            ContentTypes = GetContentTypes(contentType, otherContentTypes);
        }

        public MediaTypeCollection ContentTypes
        {
            get; set;
        }

        public int Order
        {
            get
            {
                return 0;
            }
        }

        private bool IsSubsetOfAnyContentType(string requestMediaType)
        {
            var parsedRequestMediaType = new MediaType(requestMediaType);
            for (var i = 0; i < ContentTypes.Count; i++)
            {
                var contentTypeMediaType = new MediaType(ContentTypes[i]);
                if (parsedRequestMediaType.IsSubsetOf(contentTypeMediaType))
                {
                    return true;
                }
            }
            return false;
        }

        public bool Accept(ActionConstraintContext context)
        {
            var requestAccept = context.RouteContext.HttpContext.Request.Headers[HeaderNames.Accept];
            if (StringValues.IsNullOrEmpty(requestAccept))
                return true;

            if (IsSubsetOfAnyContentType(requestAccept))
                return true;

            return false;
        }

        private MediaTypeCollection GetContentTypes(string firstArg, string[] args)
        {
            var completeArgs = new List<string>();
            completeArgs.Add(firstArg);
            completeArgs.AddRange(args);

            var contentTypes = new MediaTypeCollection();
            foreach (var arg in completeArgs)
            {
                contentTypes.Add(arg);
            }

            return contentTypes;
        }
    }

You can decorate any action with this attribute.

Note that it is easy to change and allow to specify the header you want to check and the value.

J. Doe · Accepted Answer · 2017-05-24 11:21:08Z

1

if(Request.Headers["Content-Type"] == "application/json")
{
    return OK(json);
}
else
{
    return View();
}

Thats ok?

answered May 24, 2017 at 11:21

J. Doe

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4 Comments

animalito maquina Over a year ago

i want an action selector

J. Doe Over a year ago

So create middleware with that code and return redirect to action controller

Ilya Chumakov Over a year ago

@animalitomaquina just replace return View() with return FooAction() call and you'll get the selector.

torvin Over a year ago

You are supposed to use Accept header. Content-Type is for the incoming data (e.g. format of the POST body)

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