Issue with MVC 3 model binder for propery which is a list of inherited objects - json

Question

The issue is very similar to this post
How to implement custom JsonConverter in JSON.NET to deserialize a List of base class objects?
However instead of trying to serialize a string manually we are attempting to use the model binding in MVC 3. So here is the scenario
[DataContract]
public class Company
{
[DataMember]
public List<Person> Employees { get; set; }
}
[DataContract]
public class Person
{
[DataMember]
public string FirstName { get; set; }
[DataMember]
public string LastName { get; set; }
}
[DataContract]
[KnownType(typeof(Person))]
public class Employee : Person
{
[DataMember]
public string Department { get; set; }
[DataMember]
public string JobTitle { get; set; }
}
[DataContract]
[KnownType(typeof(Person))]
public class Artist : Person
{
[DataMember]
public string Skill { get; set; }
}
public JsonResult PopulateCompany()
{
Company model = new Company();
model.Employees = new List<Person>
{
new Employee(),
new Employee(),
new Artist(),
};
return Json(model, JsonRequestBehavior.AllowGet);
// in the View the model is correctly deserialized. E.g. we can see the properties from Artist
}
public ActionResult PopulateCompany(Company model)
{
// the returned model is also being populated except the Person object is being added to the Employees and we can no longer access the properties of Artist.
return View(model);
}
Thank you.

Answer 1

The model binding process involve first initializing the model. In your case it initializes an instance of Company with a property List<Person> Employees. Based on the values that are posted back, if a key/value pair is found that matches a Person (e.g. Persons[0].FirstName: "Ian") then a new instance of Person is initialized and its properties are set and added to the collection.
The DefaultModelBinder has no way of knowing that you want to initialize a different concrete type.
The easy solution is to use a view model containing collection properties of each type (e.g. public List<Employees> Employees { get; set; }; public List<Artist> Artists { get; set; }; etc).
The alternative (difficult) solution is to create a custom ModelBinder that will generate concrete types based on values in the model. This article (the section on Abstract Model Binder) is a good start for learning how to create a custom ModelBinder