C# – WEB API Routing Error Multiple actions were found that match the request with different API paths

Figured it out.

For ApiControllers, MVC 4 uses a System.Web.Http.Dispatcher.IHttpControllerFactory and System.Web.Http.Dispatcher.IHttpControllerActivator to create the controllers. If there is no static method to register what the implementation of these they are; when they are resolved, the mvc framework looks for the implementations in the dependency resolver, and if they are not found, uses the default implementations.

I got unity resolution of controller dependencies working by doing the following:

Created a UnityHttpControllerActivator:

public class UnityHttpControllerActivator : IHttpControllerActivator
{
    private IUnityContainer _container;

    public UnityHttpControllerActivator(IUnityContainer container)
    {
        _container = container;
    }

    public IHttpController Create(HttpControllerContext controllerContext, Type controllerType)
    {
        return (IHttpController)_container.Resolve(controllerType);
    }
}

Registered that controller activator as the implementation in the unity container itself:

protected void Application_Start()
{
    // code intentionally omitted

    IUnityContainer container = BuildUnityContainer();
    container.RegisterInstance<IHttpControllerActivator>(new UnityHttpControllerActivator(container));

    ServiceResolver.SetResolver(t =>
       {
         // rest of code is the same as in question above, and is omitted.
       });
}

Your route map is probably something like this in WebApiConfig.cs:

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

But in order to have multiple actions with the same http method you need to provide webapi with more information via the route like so:

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

Notice that the routeTemplate now includes an action. Lots more info here: http://www.asp.net/web-api/overview/web-api-routing-and-actions/routing-in-aspnet-web-api

Update:

Alright, now that I think I understand what you are after here is another take at this:

Perhaps you don't need the action url parameter and should describe the contents that you are after in another way. Since you are saying that the methods are returning data from the same entity then just let the parameters do the describing for you.

For example your two methods could be turned into:

public HttpResponseMessage Get()
{
    return null;
}

public HttpResponseMessage Get(MyVm vm)
{
    return null;
}

What kind of data are you passing in the MyVm object? If you are able to just pass variables through the URI, I would suggest going that route. Otherwise, you'll need to send the object in the body of the request and that isn't very HTTP of you when doing a GET (it works though, just use [FromBody] infront of MyVm).

Hopefully this illustrates that you can have multiple GET methods in a single controller without using the action name or even the [HttpGet] attribute.