Question

I have the following ViewModels :

public class MyViewModel
{
  public BaseViewModel mySubViewModel;
}

public class ChildViewModel: BaseViewModel
{}

I then create a MyViewModel model, which contains a property of type ChildViewModel. In the View, it is displayed just fine.

Then I hit the save button to submit changes to my Model and I call the following controller:

    [HttpPost]
    public ActionResult Edit(MyViewModel model)
    {
        return null;
    }

To my surprise, the property mySubViewModel is now of type BaseViewModel instead of ChildViewModel ! I have no idea what’s going on here. What am I doing wrong ?

Answer 1

To my surprise, the property mySubViewModel is now of type
BaseViewModel instead of ChildViewModel ! I have no idea what’s going
on here. What am I doing wrong ?

You shouldn’t be surprised by that. You are doing nothing wrong. This is by design. The default model binder sees that your controller action is taking a MyViewModel argument and is attempting to bind the contents of the POST request to it. The default model binder has absolutely no way of knowing that you might have written some derived classes (such as ChildViewModel in your case) and that you want those derived classes to be used instantiated here. The fact that you have passed this concrete view model instance from your GET action to the view has no influence to the POST action. Think of it in terms of HTTP and what the default model binder sees. Think for example that this POST action could be invoked from some completely different client and not from a form submission. Could be for example an iPhone application performing an HTTP request to the POST action. See, now it makes perfect sense. The default model binder can only see the payload of the POST request and the type you specified as action argument. And that’s what he does => it instantiates this types and binds its properties from the POST payload data.

So one possibility is to write a custom model binder that will instantiate the concrete instance of your view model you wish. I have exemplified such a custom model binder at this post. In this example I have included a hidden field inside the form that will contain the concrete type of the view model that we would like to be instantiated and then the custom model binder simply uses this information to create this type at runtime.