MVC 3 Model Binding a Sub Type (Abstract Class or Interface)

Question

Say I have a Product model, the Product model has a property of ProductSubType (abstract) and we have two concrete implementations Shirt and Pants.

Here is the source:

 public class Product  {     public int Id { get; set; }      [Required]     public string Name { get; set; }      [Required]     public decimal? Price { get; set; }      [Required]     public int? ProductType { get; set; }      public ProductTypeBase SubProduct { get; set; } }  public abstract class ProductTypeBase { }  public class Shirt : ProductTypeBase {     [Required]     public string Color { get; set; }     public bool HasSleeves { get; set; } }  public class Pants : ProductTypeBase {     [Required]     public string Color { get; set; }     [Required]     public string Size { get; set; } } 

In my UI, user has a dropdown, they can select the product type and the input elements are displayed according to the right product type. I have all of this figured out (using an ajax get on dropdown change, return a partial/editor template and re-setup the jquery validation accordingly).

Next I created a custom model binder for ProductTypeBase.

 public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)  {          ProductTypeBase subType = null;          var productType = (int)bindingContext.ValueProvider.GetValue("ProductType").ConvertTo(typeof(int));          if (productType == 1)         {             var shirt = new Shirt();              shirt.Color = (string)bindingContext.ValueProvider.GetValue("SubProduct.Color").ConvertTo(typeof(string));             shirt.HasSleeves = (bool)bindingContext.ValueProvider.GetValue("SubProduct.HasSleeves").ConvertTo(typeof(bool));              subType = shirt;         }         else if (productType == 2)         {             var pants = new Pants();              pants.Size = (string)bindingContext.ValueProvider.GetValue("SubProduct.Size").ConvertTo(typeof(string));             pants.Color = (string)bindingContext.ValueProvider.GetValue("SubProduct.Color").ConvertTo(typeof(string));              subType = pants;         }          return subType;      } } 

This binds the values correctly and works for the most part, except I lose the server side validation. So on a hunch that I am doing this incorrectly I did some more searching and came across this answer by Darin Dimitrov:

ASP.NET MVC 2 - Binding To Abstract Model

So I switched the model binder to only override CreateModel, but now it doesn't bind the values.

protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)     {         ProductTypeBase subType = null;          var productType = (int)bindingContext.ValueProvider.GetValue("ProductType").ConvertTo(typeof(int));          if (productType == 1)         {             subType = new Shirt();         }         else if (productType == 2)         {             subType = new Pants();         }          return subType;     } 

Stepping though the MVC 3 src, it seems like in BindProperties, the GetFilteredModelProperties returns an empty result, and I think is because bindingcontext model is set to ProductTypeBase which doesn't have any properties.

Can anyone spot what I am doing wrong? This doesn't seem like it should be this difficult. I am sure I am missing something simple...I have another alternative in mind of instead of having a SubProduct property in the Product model to just have separate properties for Shirt and Pants. These are just View/Form models so I think that would work, but would like to get the current approach working if anything to understand what is going on...

Thanks for any help!

Update:

I didn't make it clear, but the custom model binder I added, inherits from the DefaultModelBinder

Answer

Setting ModelMetadata and Model was the missing piece. Thanks Manas!

protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)         {             if (modelType.Equals(typeof(ProductTypeBase))) {                 Type instantiationType = null;                  var productType = (int)bindingContext.ValueProvider.GetValue("ProductType").ConvertTo(typeof(int));                  if (productType == 1) {                     instantiationType = typeof(Shirt);                 }                 else if (productType == 2) {                     instantiationType = typeof(Pants);                 }                  var obj = Activator.CreateInstance(instantiationType);                 bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, instantiationType);                 bindingContext.ModelMetadata.Model = obj;                 return obj;             }              return base.CreateModel(controllerContext, bindingContext, modelType);          } 

Answer 1

This can be achieved through overriding CreateModel(...). I will demonstrate that with an example.

1. Lets create a model and some base and child classes.

public class MyModel {     public MyBaseClass BaseClass { get; set; } }  public abstract class MyBaseClass {     public virtual string MyName     {         get         {             return "MyBaseClass";         }     } }  public class MyDerievedClass : MyBaseClass {      public int MyProperty { get; set; }     public override string MyName     {         get         {             return "MyDerievedClass";         }     } } 

2. Now create a modelbinder and override CreateModel

public class MyModelBinder : DefaultModelBinder {     protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)     {         /// MyBaseClass and MyDerievedClass are hardcoded.         /// We can use reflection to read the assembly and get concrete types of any base type         if (modelType.Equals(typeof(MyBaseClass)))         {             Type instantiationType = typeof(MyDerievedClass);                             var obj=Activator.CreateInstance(instantiationType);             bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, instantiationType);             bindingContext.ModelMetadata.Model = obj;             return obj;         }         return base.CreateModel(controllerContext, bindingContext, modelType);     }  } 

3. Now in the controller create get and post action.

[HttpGet] public ActionResult Index()     {         ViewBag.Message = "Welcome to ASP.NET MVC!";          MyModel model = new MyModel();         model.BaseClass = new MyDerievedClass();          return View(model);     }      [HttpPost]     public ActionResult Index(MyModel model)     {          return View(model);     } 

4. Now Set MyModelBinder as Default ModelBinder in global.asax This is done to set a default model binder for all actions, for a single action we can use ModelBinder attribute in action parameters)

protected void Application_Start()     {         AreaRegistration.RegisterAllAreas();          ModelBinders.Binders.DefaultBinder = new MyModelBinder();          RegisterGlobalFilters(GlobalFilters.Filters);         RegisterRoutes(RouteTable.Routes);     } 

5. Now we can create view of type MyModel and a partial view of type MyDerievedClass

Index.cshtml

@model MvcApplication2.Models.MyModel  @{ ViewBag.Title = "Index"; Layout = "~/Views/Shared/_Layout.cshtml"; }  <h2>Index</h2>  @using (Html.BeginForm()) { @Html.ValidationSummary(true) <fieldset>     <legend>MyModel</legend>     @Html.EditorFor(m=>m.BaseClass,"DerievedView")     <p>         <input type="submit" value="Create" />     </p> </fieldset> } 

DerievedView.cshtml

@model MvcApplication2.Models.MyDerievedClass  @Html.ValidationSummary(true) <fieldset>     <legend>MyDerievedClass</legend>      <div class="editor-label">         @Html.LabelFor(model => model.MyProperty)     </div>     <div class="editor-field">         @Html.EditorFor(model => model.MyProperty)         @Html.ValidationMessageFor(model => model.MyProperty)     </div>  </fieldset> 

Now it will work as expected, Controller will receive an Object of type "MyDerievedClass". Validations will happen as expected.

Answer 2

I had the same problem, I ended up using MvcContrib as sugested here.

The documentation is outdated but if you look at the samples it's pretty easy.

You'll have to register your types in the Global.asax:

protected void Application_Start(object sender, EventArgs e) {     // (...)     DerivedTypeModelBinderCache.RegisterDerivedTypes(typeof(ProductTypeBase), new[] { typeof(Shirt), typeof(Pants) }); } 

Add two lines to your partial views:

@model MvcApplication.Models.Shirt @using MvcContrib.UI.DerivedTypeModelBinder @Html.TypeStamp() <div>     @Html.LabelFor(m => m.Color) </div> <div>     @Html.EditorFor(m => m.Color)     @Html.ValidationMessageFor(m => m.Color) </div> 

Finally, in the main view (using EditorTemplates):

@model MvcApplication.Models.Product @{     ViewBag.Title = "Products"; } <h2>     @ViewBag.Title</h2>  @using (Html.BeginForm()) {     <div>         @Html.LabelFor(m => m.Name)     </div>     <div>         @Html.EditorFor(m => m.Name)         @Html.ValidationMessageFor(m => m.Name)     </div>     <div>         @Html.EditorFor(m => m.SubProduct)     </div>     <p>         <input type="submit" value="create" />     </p> }