Multiple actions were found that match the request [duplicate]

Answer 1

We can also create custom action selector for Api Controllers as follows, so that that can freely work with complex types with traditional "GET,POST,PUT,DELETE":

 class ApiActionSelector : IHttpActionSelector
    {
        private readonly IHttpActionSelector defaultSelector;

        public ApiActionSelector(IHttpActionSelector defaultSelector)
        {
            this.defaultSelector = defaultSelector;
        }
        public ILookup<string, HttpActionDescriptor> GetActionMapping(HttpControllerDescriptor controllerDescriptor)
        {
            return defaultSelector.GetActionMapping(controllerDescriptor);
        }
        public HttpActionDescriptor SelectAction(HttpControllerContext controllerContext)
        {
            // Get HttpMethod from current context
            HttpMethod httpMethod = controllerContext.Request.Method;

            // Set route values for API
            controllerContext.RouteData.Values.Add("action", httpMethod.Method);

            // Invoke Action
            return defaultSelector.SelectAction(controllerContext);
        }
    }

And we can register the same in WebApiConfig as :

 config.Services.Replace(typeof(IHttpActionSelector), new
 ApiActionSelector(config.Services.GetActionSelector()));

May help users who are running this kind of issue.

Answer 2

The problem is in the order that the routes are loaded. If they are like this:

// A route that enables RPC requests
routes.MapHttpRoute(
    name: "RpcApi",
    routeTemplate: "rpc/{controller}/{action}",
    defaults: new { action = "Get" }
);

// Default route
routes.MapHttpRoute(
    name: "Default",
    routeTemplate: "{controller}/{id}",
   defaults: new { id =  RouteParameter.Optional }
);

It will work fine. First the request will be mapped against the RPC, then against the controller (thaty may have or not an Id).