ASP.NET model binding to base type

I have a BaseViewModel that my View Models all inherit from.

public class MagazineViewModel : BaseOutputViewMode
{
    public string TitleOfPublication { get; set; }
}

In my controller I use a factory method to give the corret View Model back based on an input:

// e.g. viewModel contains an instance of MagazineViewModel 
BaseOutputViewModel viewModel = BaseOutputViewModel.GetOutputViewModel(output);

When I use TryUpdateModel to try and bind to a FormCollection which I know contains a "TitleOfPublication" key, its never set in my view model:

if (!TryUpdateModel(viewModel, form))

I think this is something to do with the DefaultModelBinder using the BaseOutputViewModel to bind FormCollection keys to - it doesn't contain a "TitleOfPublication", the derived MagazineViewModel does.

I'm trying to roll my own model binder, to ove开发者_Python百科rride the DefaultModelBinder's BindModel behavior. Its all wired in correctly and I can debug into it straight after the TryUpdateModel call:

 public class TestModelBinder : DefaultModelBinder, IFilteredModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        // Tried the following without success ....
        // 1. Quick hardcoded test
        // bindingContext.ModelType = typeof(MagazineViewModel);
        // 2. Set ModelMetadata, hardcoded test again
        // bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, typeof(MagazineViewModel));
        // 3. Replace the entire context
        // ModelBindingContext context2 = new ModelBindingContext();
        // context2.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, typeof(MagazineViewModel));
        // context2.ModelName = bindingContext.ModelName;
        // context2.ModelState = bindingContext.ModelState;            
        // context2.ValueProvider = bindingContext.ValueProvider;
        // bindingContext = context2;
    }
}

But I'm not sure how to work with the bindingContext? What needs to be updated so that I can tell the DefaultModelBinder to bind using the derived View Model properties? Or have I just totally mis-understood this!

I did try overriding CreateModel - much like the DerivedTypeModelBinder in MvcContrib, but I think because I'm giving the binder an instance of a model to work with, CreateModel is never called. Used Reflector on the Mvc DLL, theres a "BindComplexModel" that calls CreateModel only if the model is null:

if (model == null)
{
    model = this.CreateModel(controllerContext, bindingContext, modelType);
}

Any pointers greatfully received!

Cheers

OK - finally got to the bottom of this! In actual fact there was nothing wrong with my model binder, the problem ultimately led back to a couple of input tags that had no name/id:

<input id="" name="" type="text">

The crux was this test in DefaultModelBinder:

// Simple model = int, string, etc.; determined by calling TypeConverter.CanConvertFrom(typeof(string))
// or by seeing if a value in the request exactly matches the name of the model we're binding.
// Complex type = everything else.
if (!performedFallback) {
     ValueProviderResult vpResult =
            bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
            if (vpResult != null) {
                return BindSimpleModel(controllerContext, bindingContext, vpResult);
            }
        }

With no id/name, the form collection has a key of "" which means that the GetValue correctly returned the value for that field, continuing to bind as a simple model.

When an id/name are added, the form collection contains no key of "", (which is now the name of my model as we're using TryUpdateModel). This meant the DefaultModelBinder correctly treated my model as complexm successfully binding properties in my derived type!

Cheers