ASP.NET MVC controller actions with custom parameter conversion?

Question

I want to set up a ASP.NET MVC route that looks like:

routes.MapRoute(
  "Default", // Route name
  "{controller}/{action}/{idl}", // URL with parameters
  new { controller = "Home", action = "Index", idl = UrlParameter.Optional } // Parameter defaults
);

That routes requests that look like this...

Example/GetItems/1,2,3

...to my controller action:

public class ExampleController : Controller
{
    public ActionResult GetItems(List<int> id_list)
    {
        return View();
    }
}

The question is, what do I set up to transform the idl url parameter from a string into List<int> and call the appropriate controller action?

I have seen a related question here that used OnActionExecuting to preprocess a string, but did not change the type. I don't think that will work for me here, because when I override OnActionExecuting in my controller and inspect the ActionExecutingContext parameter, I see that the ActionParameters dictionary already has an idl key with a null value- presumably, an attempted cast from string to List<int> ... this is the part of the routing I want to be in control of.

Is this possible?

Answer 1

A nice version is to implement your own Model Binder. You can find a sample here

I try to give you an idea:

public class MyListBinder : IModelBinder
{   
     public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
     {   
        string integers = controllerContext.RouteData.Values["idl"] as string;
        string [] stringArray = integers.Split(',');
        var list = new List<int>();
        foreach (string s in stringArray)
        {
           list.Add(int.Parse(s));
        }
        return list;  
     }  
}


public ActionResult GetItems([ModelBinder(typeof(MyListBinder))]List<int> id_list) 
{ 
    return View(); 
} 

Answer 2

Like slfan's says, a custom model binder is the way to go. Here's a another approach from my blog which is generic and supports multiples data types. It also elegantly falls back to default the model binding implementation:

public class CommaSeparatedValuesModelBinder : DefaultModelBinder
{
    private static readonly MethodInfo ToArrayMethod = typeof(Enumerable).GetMethod("ToArray");

    protected override object GetPropertyValue(ControllerContext controllerContext, ModelBindingContext bindingContext, System.ComponentModel.PropertyDescriptor propertyDescriptor, IModelBinder propertyBinder)
    {
        if (propertyDescriptor.PropertyType.GetInterface(typeof(IEnumerable).Name) != null)
        {
            var actualValue = bindingContext.ValueProvider.GetValue(propertyDescriptor.Name);

            if (actualValue != null && !String.IsNullOrWhiteSpace(actualValue.AttemptedValue) && actualValue.AttemptedValue.Contains(","))
            {
                var valueType = propertyDescriptor.PropertyType.GetElementType() ?? propertyDescriptor.PropertyType.GetGenericArguments().FirstOrDefault();

                if (valueType != null && valueType.GetInterface(typeof(IConvertible).Name) != null)
                {
                    var list = (IList)Activator.CreateInstance(typeof(List<>).MakeGenericType(valueType));

                    foreach (var splitValue in actualValue.AttemptedValue.Split(new[] { ',' }))
                    {
                        list.Add(Convert.ChangeType(splitValue, valueType));
                    }

                    if (propertyDescriptor.PropertyType.IsArray)
                    {
                        return ToArrayMethod.MakeGenericMethod(valueType).Invoke(this, new[] { list });
                    }
                    else
                    {
                        return list;
                    }
                }
            }
        }

        return base.GetPropertyValue(controllerContext, bindingContext, propertyDescriptor, propertyBinder);
    }
}