ASP.NET: Modelbinder only delegating

Question

I'm facing a problem ith a custom modelbinder.

I have two models (inheriting from a base class) that are displayed by EditorTemplates.

Base-Class:

public abstract class QuestionAnswerInputModel {
    public Guid QuestionId {
        get; set;
    }
}

Modelclass 1:

public class RatingQuestionInputModel : QuestionAnswerInputModel{
    [Required]
    [Range(1,4)]
    public int? Rating { get; set; }
}

Modelclass 2:

public class FreeTextQuestionInputModel: QuestionAnswerInputModel{
    [Required]
    public string FreeText { get; set; }
}

To get it bound I implemented a custom modelbinder:

public class QuestionAnswerModelBinder : DefaultModelBinder {
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {

        QuestionAnswerInputModel model;

        if ((typeof(QuestionAnswerInputModel) != bindingContext.ModelType)) {
            return null;
        }

        ModelBindingContext context = new ModelBindingContext(bindingContext);

        Type typeOfModel;

        string prefix = bindingContext.ModelName;
        if (bindingContext.ValueProvider.ContainsPrefix(prefix + "." + new FreeTextQuestionInputModel().GetPropertyName(m => m.FreeText))) {
            typeOfModel = typeof(FreeTextQuestionInputModel);
        } else if (bindingContext.ValueProvider.ContainsPrefix(prefix + "." + new RatingQuestionInputModel().GetPropertyName(m => m.Rating))) {
            typeOfModel = typeof(RatingQuestionInputModel);
        } else {
            return null;
        }

        context.ModelMetadata = new ModelMetadata(new DataAnnotationsModelMetadataProvider(), bindingContext.ModelMetadata.ContainerType, null, typeOfModel, bindingContext.ModelName);
        return base.BindModel(controllerContext, context);
    }
}

All in all it works great, BUT the values fpr properties of the models (QuestionId and Rating/Freetext) are not set? Can anyone tell me why? What am I doing wrong?

I also tried to call

new DefaultModelBinder().BindModel(controllerContext, context)

but the result is the same. Correctly instantiated objects but properties are not set.

---

UPDATE:

I now tried to override just the CreateModel-Methode of the DefaultBinder like in this post MVC 3 Model Binding a Sub Type (Abstract Class or Interface).

protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType) {

        if ((typeof(QuestionAnswerInputModel) != bindingContext.ModelType)) {
            return null;
        }

        string prefix = bindingContext.ModelName;
        if (bindingContext.ValueProvider.ContainsPrefix(prefix + "." + new FreeTextQuestionInputModel().GetPropertyName(m => m.FreeText))) {
            return new FreeTextQuestionInputModel();
        } else if (bindingContext.ValueProvider.ContainsPrefix(prefix + "." + new RatingQuestionInputModel().GetPropertyName(m => m.Rating))) {
            return new RatingQuestionInputModel();
        } else {
            return null;
        }
    }

The model is still instantiated correctly. The problem now is, that only the properties of the base class are set.

Answer 1

After some discussion with @macpak I found the solution. This works great for me:

protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType) {

    if ((typeof(QuestionAnswerInputModel) != bindingContext.ModelType)) {
        return null;
    }

    string prefix = bindingContext.ModelName;

    QuestionAnswerInputModel obj;
    if (bindingContext.ValueProvider.ContainsPrefix(prefix + "." + new FreeTextQuestionInputModel().GetPropertyName(m => m.FreeText))) {
        obj = new FreeTextQuestionInputModel();
    } else if (bindingContext.ValueProvider.ContainsPrefix(prefix + "." + new RatingQuestionInputModel().GetPropertyName(m => m.Rating))) {
        obj = new RatingQuestionInputModel();
    } else {
        return null;
    }

    bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, obj.GetType());
    bindingContext.ModelMetadata.Model = obj;

    return obj;
}

I just have to override the CreateModel-Methode. Thanks to @MacPak!