Register ViewModels with Autofac by overriding DefaultModelBinder

Autofac is a great IoC framework and we all know that. I however had a minor problem when registering ViewModels for my ASP.Net MVC application. If you register your viewmodels like a regular assembly or regular types, it will work perfect only if your viewmodel does not have any dependency or it has a parameter less constructor.

Here’s the example:

public class CustomerController : Controller
    {
        public ActionResult Index(CustomerViewModel viewModel)
        {
            return View(viewModel);
        }

        
    }


    public class CustomerViewModel
    {
        ICustomerRepository repository;
        public CustomerViewModel(ICustomerRepository repository)
        {
            this.repository = repository;
            GetAllCustomers();
        }

        private void GetAllCustomers()
        {
            //Do something
        }
    }

if you’ve registered CustomerViewModel as following then this will throw a runtime error.

builder.RegisterType<CustomerViewModel>();

 

This does not work because Autofac will not intercept when CustomerViewModel will be created by DefaultModelBinder.

Here’s a workaround:

Create a new CustomeModelBinder class which will inherit from DefaultModelBinder and override the CreateModel method. Using DependencyResolver class, Autofac will now intercept and resolves dependency of viewmodels.

public class CustomModelBinder<T> : DefaultModelBinder where T : class
    {
        protected override object CreateModel(ControllerContext controllerContext,
            ModelBindingContext bindingContext, Type modelType)
        {
            var item = DependencyResolver.Current.GetService<T>();

            if (item != null)
            {
                return item;
            }
            throw new ArgumentException(string.Format("model type {0} is not registered",                            modelType.Name));
        }
    }

 

Now a simple extension method to Register all viewmodels (assuming the convention is have all viewmodel class name ends with “ViewModel”)

 

public static void RegisterViewModels(this ContainerBuilder builder)
        {
            var types = System.Reflection.Assembly.GetExecutingAssembly().GetTypes().Where(x => x.Name.EndsWith("ViewModel", StringComparison.OrdinalIgnoreCase));
            types.ToList().ForEach(modelType =>
            {
                Type modelBinderType = typeof(CustomModelBinder<>);
                Type[] typeArgs = { modelType };
                Type genType = modelBinderType.MakeGenericType(typeArgs);
                object viewModel = Activator.CreateInstance(genType);
                ModelBinders.Binders.Add(modelType, (IModelBinder)viewModel);
                builder.RegisterType(modelType).InstancePerDependency();
            });
        }

Alternatively, if all your viewmodels inherit from a common base class (say, BaseViewModel) then following extension can be handy.

public static void RegisterBaseViewModelDescendants<T>(this ContainerBuilder builder) where T : BaseViewModel
        {
            var types = System.Reflection.Assembly.GetExecutingAssembly().GetTypes().Where(x => x.IsSubclassOf(typeof(T)));
            types.ToList().ForEach(modelType =>
            {
                Type modelBinderType= typeof(IocModelBinder<>);
                Type[] typeArgs = { modelType };
                Type genType = modelBinderType.MakeGenericType(typeArgs);
                object viewmodel = Activator.CreateInstance(genType);
                ModelBinders.Binders.Add(modelType, (IModelBinder)viewmodel);
                builder.RegisterType(modelType).InstancePerDependency();
            });
        }