asp.net MVC model binding error "Cannot create an instance of Interface"

Question

I Have a ViewModel that has an interface as a property. When I submit the page, I got the "Cannot create an instance of Interface" error.

The ViewModel is like this:

public class PlanoPagamentoViewModel
{
     //some properties
     public IPlanoPagamentosParcelas PlanoPagamentosParcelas { get; set; }     
}

There're two classes that implement this Interface. The corresponding ViewModels are dinamically loaded with a PartialView, depending on the option that is selected.

public class PlanoPagamentoCartaoViewModel : IPlanoPagamentosParcelas
{
   //some properties
}

public class PlanoPagamentoCrediarioViewModel : IPlanoPagamentosParcelas
{
   //some properties
}

I did a research and I found that the need of creating a custom model binding, and I did that:

public class PlanoPagamentoParcelasBinder : DefaultModelBinder
{
    protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
    {
        var type = typeof(PlanoPagamentoCartaoViewModel);
        var model = Activator.CreateInstance(type);
        bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, type);

        return model;
    }
}

And add this new custom binding in Global.asax, Application_Start method:

ModelBinders.Binders.Add(typeof(IPlanoPagamentosParcelas), new PlanoPagamentoParcelasBinder());  

It works well for PlanoPagamentoCartaoViewModel, but I would need to have another custom bindings for the PlanoPagamentoCrediarioViewModel, but I can't just add a new ModelBinders.Binders.Add with the same Key (IPlanoPagamentosParcelas) because there's already one key with this type.

So, is there any other approach to create a custom model binding for ViewModels that implement the same interface?

Answer 1

I believe that the solution is as follow:

protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
{
        var typeValueProvider = bindingContext.ValueProvider.GetValue("Type");

        var type = (int)typeValueProvider.ConvertTo(typeof(int));

        Type instanceType = null;

        switch (type)
        {
            case 1:
                instanceType = typeof(PlanoPagamentoCartaoViewModel );
                break;

            case 2:
                instanceType = typeof(PlanoPagamentoCrediarioViewModel );
                break;
        }

        if (!typeof(IPlanoPagamentosParcelas).IsAssignableFrom(instanceType))
        {
            throw new InvalidOperationException("Bad Type");
        }
        var model = Activator.CreateInstance(instanceType);
        bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, instanceType);
        return model;
}

In the above code var typeValueProvider = bindingContext.ValueProvider.GetValue("Type"); Type would be something to decide which concrete class to instantiate. I´ve tested this solution and it worked for me. It can be improved to be more extensible (removing the switch for something else maybe) but I´ve just tried to make it work.

Here is from where I based my question (maybe you can get more information from there) Polymorphic model binding and below is the code that I created to simulate this scenario (just in case of doubts):

MODELS:

public class NewVehicleViewModel
{
    public string Type { get; set; }

    public VehicleViewModel Vehicle { get; set; }
}

public interface VehicleViewModel
{
    string Name { get; set; }
    string Color { get; set; }
}

public class CarViewModel : VehicleViewModel
{
    public string Color { get; set; }

    public string Name { get; set; }

    public string Brand { get; set; }
}

public class TankViewModel : VehicleViewModel
{
    public string Color { get; set; }

    public string Name { get; set; }

    public string Weapon { get; set; }
}

BINDER:

public class VehicleBinder : DefaultModelBinder
{
    protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
    {
        var typeValueProvider = bindingContext.ValueProvider.GetValue("Type");

        var type = (int)typeValueProvider.ConvertTo(typeof(int));

        Type instanceType = null;

        switch (type)
        {
            case 1:
                instanceType = typeof(CarViewModel);
                break;

            case 2:
                instanceType = typeof(TankViewModel);
                break;
        }

        if (!typeof(VehicleViewModel).IsAssignableFrom(instanceType))
        {
            throw new InvalidOperationException("Bad Type");
        }
        var model = Activator.CreateInstance(instanceType);
        bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, instanceType);
        return model;
    }
}

CONTROLLER:

public class VehiclesController : Controller
{
    // GET: Vehicles
    public ActionResult Index()
    {
        return View();
    }

    [HttpPost]
    public ActionResult Create(NewVehicleViewModel vm)
    {
        return View();
    }
}

GLOBAL.ASAX:

    protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();
        FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
        RouteConfig.RegisterRoutes(RouteTable.Routes);
        BundleConfig.RegisterBundles(BundleTable.Bundles);
        ModelBinders.Binders.Add(typeof(VehicleViewModel), new VehicleBinder());
    }

VIEW (Index.cshtml):

@{
    ViewBag.Title = "New Vehicle";
}


@using (Html.BeginForm("Create", "Vehicles"))
{
    <label>Type</label>
    <input name="type" type="text" />

    <label >Name</label>
    <input name="Vehicle.Name" type="text"/>

    <label>Color</label>
    <input name="Vehicle.Color" type="text" />

    <label>Weapon</label>
    <input name="Vehicle.Weapon" type="text" />

    <label>Brand</label>
    <input name="Vehicle.Brand" type="text" />

    <input type="submit" />
}

Regards.