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\" r

Answer 1

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.

    /// 

    /// A filter that specifies the supported response content types. The request accept header is used to determine if it is a valid action
    /// 
    [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();
            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.