Not understanding the correct way to route my action - asp.net-mvc

Question

This is driving me crazy. This is my controller action:
[HttpGet]
public List<Product> RetrieveByIds(int[] ids)
{
Product p = new Product();
//return p.RetrieveByIds(ids);
return new List<Product>();
}
This is my routing file:
public static void Register(HttpConfiguration config)
{
config.Routes.MapHttpRoute(
name: "DefaultApi",
routeTemplate: "api/{controller}/{id}",
defaults: new { id = RouteParameter.Optional }
);
config.Routes.MapHttpRoute(
name: "ActionApi",
routeTemplate: "api/{controller}/{action}/{id}",
defaults: new { id = RouteParameter.Optional }
);
config.Routes.MapHttpRoute(
name: "ActionApiMultipleIds",
routeTemplate: "api/{controller}/{action}/{ids}",
defaults: new { ids = RouteParameter.Optional }
);
}
And I keep getting this error when I try to test it using the RestConsole:
{
"Message": "The request is invalid.",
"MessageDetail": "The parameters dictionary contains a null entry for parameter 'id' of non-nullable type 'System.Int32' for method 'System.Collections.Generic.List`1[GrownHome.Web.Models.Product] RetrieveAllByUserId(Int32)' in 'GrownHome.Web.Controllers.Api.ProductController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter."
}
I keep starring at my screen fiddling around with the code and I am getting nowhere. I don't have to explain to you guys how frustrating this is :)))
I run into these kind of routing issues every now and again and I can't seem to get my head around these kind of issues. Some insight would be appriciated very much!
Thanks!

Answer 1

The framework tries to match a URL to a route based on the order the routes are listed in your Register method, so your third route will actually never be used because the first two routes are the default "catch-all" routes. This is why you are receiving the error saying that parameter id is null.
The general rule of thumb is to place more specific routes before more general routes to ensure the routing framework matches a URL to a route correctly. So in your case your custom route needs to be moved ahead of the two default routes:
public static void Register(HttpConfiguration config)
{
config.Routes.MapHttpRoute(
name: "ActionApiMultipleIds",
routeTemplate: "api/{controller}/{action}/{ids}",
defaults: new { ids = RouteParameter.Optional }
);
config.Routes.MapHttpRoute(
name: "DefaultApi",
routeTemplate: "api/{controller}/{id}",
defaults: new { id = RouteParameter.Optional }
);
config.Routes.MapHttpRoute(
name: "ActionApi",
routeTemplate: "api/{controller}/{action}/{id}",
defaults: new { id = RouteParameter.Optional }
);
}

Answer 2

Your third route will never be reaced because the one before is catching all the request that have the same formatting. You probably need to add some constraints.