asp.net-mvc-4 – 自定义多态模型绑定程序不绑定派生类型的属性

public class LocationModalBinder : DefaultModelBinder
{
    protected override object CreateModel(ControllerContext controllerContext,ModelBindingContext bindingContext,Type modelType)
    {
        var type = bindingContext.ModelName + "." + "type";

        Type typeToInstantiate;

        switch ((string) bindingContext.ValueProvider.GetValue(type).RawValue)
        {
            case "store":
            {
                typeToInstantiate = typeof (Store);
                break;
            }
            case "billing":
            {
                typeToInstantiate = typeof(LocationReference);
                break;
            }
            case "alternate":
            {
                typeToInstantiate = typeof(Address);
                break;
            }
            default:
            {
                throw new Exception("Unknown location identifier.");
            }
        }

        return base.CreateModel(controllerContext,bindingContext,typeToInstantiate);
    }
}

问题是它不绑定子类型的属性.仅限基本类型位置上的属性.为什么是这样？