New Post: Easiest and most elegant way to inject custom IActionResultConverter instances

It would have been easier had IActionResultConverter been replaceable as a Service....but currently its not. Following is a way I have tried and this seems to work and its similar to what you mentioned above. I use a wrapper ReflectedHttpActionDescriptor to which I supply the default implementation and I just provide my custom implementation for ResultConverter only and for the remaining delegate it to the original one.

 public class CustomActionSelector : ApiControllerActionSelector
    {
        public override HttpActionDescriptor SelectAction(HttpControllerContext controllerContext)
        {
            return new WrapperReflectedHttpActionDescriptor(base.SelectAction(controllerContext));
        }

        public override ILookup<string, HttpActionDescriptor> GetActionMapping(HttpControllerDescriptor controllerDescriptor)
        {
            //Wrap the original action descriptor into Wrapper action descriptor
            return base.GetActionMapping(controllerDescriptor)
                                .SelectMany(g => g, (g, v) => new KeyValuePair<string, HttpActionDescriptor>(g.Key, new WrapperReflectedHttpActionDescriptor(v)))
                                .ToLookup(kvp => kvp.Key, kvp => kvp.Value);
        }
    }

    public class WrapperReflectedHttpActionDescriptor : ReflectedHttpActionDescriptor
    {
        private HttpActionDescriptor _original;

        public WrapperReflectedHttpActionDescriptor(HttpActionDescriptor original)
        {
            this.Configuration = original.Configuration;
            this.ControllerDescriptor = original.ControllerDescriptor;
           
            this._original = original;
        }

        public override HttpActionBinding ActionBinding
        {
            get
            {
                return this._original.ActionBinding;
            }
            set
            {
                this._original.ActionBinding = value;
            }
        }

        public override string ActionName
        {
            get
            {
                return this._original.ActionName;
            }
        }

        public override System.Threading.Tasks.Task<object> ExecuteAsync(HttpControllerContext controllerContext, IDictionary<string, object> arguments, System.Threading.CancellationToken cancellationToken)
        {
            return this._original.ExecuteAsync(controllerContext, arguments, cancellationToken);
        }

        public override System.Collections.ObjectModel.Collection<T> GetCustomAttributes<T>()
        {
            return this._original.GetCustomAttributes<T>();
        }

        public override System.Collections.ObjectModel.Collection<System.Web.Http.Filters.FilterInfo> GetFilterPipeline()
        {
            return this._original.GetFilterPipeline();
        }

        public override System.Collections.ObjectModel.Collection<System.Web.Http.Filters.IFilter> GetFilters()
        {
            return this._original.GetFilters();
        }

        public override System.Collections.ObjectModel.Collection<HttpParameterDescriptor> GetParameters()
        {
            return this._original.GetParameters();
        }

        public override System.Collections.Concurrent.ConcurrentDictionary<object, object> Properties
        {
            get
            {
                return this._original.Properties;
            }
        }

        public override IActionResultConverter ResultConverter
        {
            get
            {
                //provide custom one
                return new CustomActionResultConverter();
            }
        }

        public override Type ReturnType
        {
            get
            {
                return this._original.ReturnType;
            }
        }

        public override System.Collections.ObjectModel.Collection<HttpMethod> SupportedHttpMethods
        {
            get
            {
                return this._original.SupportedHttpMethods;
            }
        }
    }

    public class CustomActionResultConverter : IActionResultConverter
    {
        public HttpResponseMessage Convert(HttpControllerContext controllerContext, object actionResult)
        {
           HttpResponseMessage response = new HttpResponseMessage();
            response.Content = new StringContent("Hello");
            return response;
        }
    }