Struct in MVC4 Web API causes route to break - asp.net-mvc-4

Question

I have a simple controller with a parameter-less Get and a Get that takes in an id.
public class BooksController : ApiController
{
public IEnumerable<string> Get()
{
return new string[] { "value1", "value2" };
}
public string Get(Identity id)
{
return "value";
}
}
Identity is a custom struct that, simplified, looks like this:
public struct Identity
{
// ....
public Identity(Guid value)
{
internalValue = value;
}
}
However, I receive an error stating that multiple actions were found that match the request if I attempt to navigate to either endpoint. The BooksController works just fine if the Get for a single resource takes in a Guid rather than my Identity struct.
My custom model binder and wire-up:
public class IdentityModelBinder : IModelBinder
{
public bool BindModel(HttpActionContext actionContext, ModelBindingContext bindingContext)
{
Identity identity = new Identity(Guid.Parse(bindingContext.ValueProvider.GetValue(bindingContext.ModelName).AttemptedValue));
bindingContext.Model = identity;
return true;
}
}
GlobalConfiguration.Configuration.BindParameter(typeof(Identity), new IdentityModelBinder());
Note, the above binds fine if any parameters are added to the parameter-less Get. That is, if I change my BooksController to:
public class BooksController : ApiController
{
public IEnumerable<string> Get(int page, string searchTerm)
{
return new string[] { "value1", "value2" };
}
public string Get(Identity id)
{
return "value";
}
}
My route configuration is just the out of the box example:
public static void Register(HttpConfiguration config)
{
config.Routes.MapHttpRoute(
name: "DefaultApi",
routeTemplate: "api/{controller}/{id}",
defaults: new { id = RouteParameter.Optional }
);
}
Then I can correctly navigate to both endpoints. How do I go about setting up my BooksController to allow both the parameter-less Get and a Get that takes in my custom Identity?

Answer 1

I think I've found the source of my troubles and I don't see a way around it.
Within the Web API ApiControllerActionSelector, specifically in the ActionCacheSelectorItem.ctor, the logic for resolving action parameters is:
_actionParameterNames.Add(
actionDescriptor,
actionBinding.ParameterBindings
.Where(binding => !binding.Descriptor.IsOptional && TypeHelper.IsSimpleUnderlyingType(binding.Descriptor.ParameterType) && binding.WillReadUri())
.Select(binding => binding.Descriptor.Prefix ?? binding.Descriptor.ParameterName).ToArray());
Unfortunately, this means that only simple types (primitive, datetime, guid, etc) will be included in the parameter list. I can get my struct to show up in the parameter list if I extend the filter logic above to also consider if the parameter can be converted from a string like so:
_actionParameterNames.Add(
actionDescriptor,
actionBinding.ParameterBindings
.Where(binding => !binding.Descriptor.IsOptional &&
(TypeHelper.IsSimpleUnderlyingType(binding.Descriptor.ParameterType) ||
TypeHelper.HasStringConverter(binding.Descriptor.ParameterType)) &&
binding.WillReadUri())
.Select(binding => binding.Descriptor.Prefix ?? binding.Descriptor.ParameterName).ToArray());
I am hoping I'm wrong. Will come back and mark this as the answer after the cooldown period if it turns out I'm stuck.