How to model bind a class that implements an interface?

Question

The model binding worked fine until i implemented interfaces on top of the following classes: ``` public class QuestionAnswer : IQuestionAnswer { public Int32 Row_ID { get; set; } public Int32 Column_ID { get; set; } public String Value { get; set; } } public class HiddenAnswer : IHiddenAnswer { public Int32 Hidden_Field_ID { get; set; } public String Hidden_Field_Value { get; set; } } public class SurveyAnswer : ISurveyAnswer { public string SessionID { get; set; } public List QuestionAnswerList { get; set; } public List HiddenAnswerList { get; set; } public SurveyAnswer() { QuestionAnswerList = new List(); HiddenAnswerList = new List(); } } ``` Now that the interfaces are there, i get a `500 (Internal Server Error)` The javascript that i use to model bind is the following: ``` $('#submitbutton').click(function () { var answers = new Array(); var hiddenfields = new Array(); var formname = "#" + $("#formname").val(); $(':…

Accepted Answer

what am i doing wrong?

You cannot expect the model binder to know that when it encounters the `IQuestionAnswer` interface on your `SurveyAnswer` view model it should use the `QuestionAnswer` type. It's nice that you have declared this implementation of the interface but the model binder has no clue about it.

So you will have to write a custom model binder for the `IQuestionAnswer` interface (same for the `IHiddenAnswer` interface) and indicate which implementation do you want to be used:

```
public class QuestionAnswerModelBinder : DefaultModelBinder
{
    protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
    {
        var type = typeof(QuestionAnswer);
        var model = Activator.CreateInstance(type);
        bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, type);
        return model;
    }
}
```

which will be registered in your `Application_Start`:

```
ModelBinders.Binders.Add(typeof(IQuestionAnswer), new QuestionAnswerModelBinder());
```

How to model bind a class that implements an interface?

Solution

Problem