ASP.NET MVC 2 - Binding To Abstract Model

Question

If i have the following strongly-typed view:

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<XXX.DomainModel.Core.Locations.Location>" %>

Where Location is an abstract class.

And i have the following Controller, which accepts a strongly-typed Model via a POST:

[HttpPost]
public ActionResult Index(Location model)

I get a runtime error stating "Cannot Create Abstract Class

Which of course makes sense. However - i'm not sure what the best solution is here.

I have many concrete types (around 8), and this is a view where you can only edit properties of the abstract class.

What i've tried to do is create overloads for all the different concrete types, and perform my logic in a common method.

[HttpPost]
public ActionResult Index(City model)
{
   UpdateLocationModel(model);
   return View(model);
}

[HttpPost]
public ActionResult Index(State model)
{
   UpdateLocationModel(model);
   return View(model);
}

etc etc

And then:

[NonAction]
private void UpdateLocationModel (Location model)
{
   // ..snip - update model
}

But this doesn't work either , MVC complains the action methods are ambiguous (also makes sense).

What do we do? Can we simply not bind to an abstract model?

Answer 1

How about writing a custom model binder for this abstract class:

public class CustomBinder : DefaultModelBinder
{
    protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
    {
        // TODO: based on some request parameter choose the proper child type
        // to instantiate here
        return new Child();
    }
}

This make sense only if you have a form where input elements are inserted dynamically based on some user action. In this case you need to pass some additional parameter to indicate which concrete class you need. Otherwise I would stick to concrete view models as action parameters.

Answer 2

You can also build a generic ModelBinder that works for all of your abstract models. My solution requires you to add a hidden field to your view called ' ModelTypeName ' with the value set to the name of the concrete type that you want. However, it should be possible to make this thing smarter and pick a concrete type by matching type properties to fields in the view.

In your Global.asax.cs file in Application_Start() :

ModelBinders.Binders.DefaultBinder = new CustomModelBinder();

CustomModelBinder:

public class CustomModelBinder2 : DefaultModelBinder 
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var modelType = bindingContext.ModelType;
        if (modelType.IsAbstract)
        {
            var modelTypeValue = controllerContext.Controller.ValueProvider.GetValue("ModelTypeName");
            if (modelTypeValue == null)
                throw new Exception("View does not contain ModelTypeName");

            var modelTypeName = modelTypeValue.AttemptedValue;

            var type = modelType.Assembly.GetTypes().SingleOrDefault(x => x.IsSubclassOf(modelType) && x.Name == modelTypeName);

            if (type != null)
            {
                var instance= bindingContext.Model ?? base.CreateModel(controllerContext, bindingContext, type);
                bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => instance, type);
            }
        }
        return base.BindModel(controllerContext, bindingContext);
    }
}

Answer 3

Just to throw it out there - I'm very much interested in what others might answer, but this is what I ended up doing in the case where I had a similar situation;

Basically, I did not use the model class as a parameter in the Action method, instead passing in FormCollection and testing a couple known discriminators to figure out which type to create/edit, then used TryUpdateModel from there.

It seemed there might be a better way, but I'd never gotten around to thinking about it more.