Question

I am trying to make a binder for an abstract class. The binder decides which implementation of the class to use.

public abstract class Pet
{
    public string name { get; set; }
    public string species { get; set; }
    abstract public string talk { get; }
}

public class Dog : Pet
{
    override public string talk { get { return "Bark!"; } }
}
public class Cat : Pet
{
    override public string talk { get { return "Miaow."; } }
}
public class Livestock : Pet
{
    override public string talk { get { return "Mooo. Mooo. Fear me."; } }
}

So I have a controller which takes a Pet, the binder decides (depending on the species string) if it is a Dog, Cat or Livestock.

public class PetBinder : IModelBinder
{
    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var values = (ValueProviderCollection)bindingContext.ValueProvider;
        var name = (string)values.GetValue("name").ConvertTo(typeof(string));
        var species = (string)values.GetValue("species").ConvertTo(typeof(string));
        if (species == "dog")
        {
            return new Dog { name = name, species = "dog" };
        }
        else if (species == "cat")
        {
            return new Cat { name = name, species = "cat" };
        }
        else
        {
            return new Livestock { name = name, species = species };
        }
    }
}


public class HomeController : Controller
{
    public JsonResult WorksFine(Pet pet)
    {
        return Json(pet);
    }

    public JsonResult DoesntWork(List<Pet> pets)
    {
        return Json(pets);
    }
}

This works well, but as soon as the pet is in another structure (like List<Pet> or another object), I get a NullReferenceException (on the line var name = (string)values.GetValue("name").ConvertTo(typeof(string));
in the PetBinder). What am I doing wrong?

I added a Person class to test. It also gave me a NullReferenceException.

public class Person
{
    public string name { get; set; }
    public Pet pet { get; set; }
}
public class HomeController : Controller
{
    public JsonResult PersonAction(Person p)
    {
        return Json(p);
    }
}

ccurrens said the reason var name = (string)values.GetValue("name").ConvertTo(typeof(string));
returned null was because it couldn’t get the values from a list.

I see they are named [n].name and [n].species when in a List<Pet>, but when in the Person object they are named pet.name and pet.species and when they are in a single Pet, they are just named name and species.

Solution

To get the parameter names with the right prefix ([n] or pet or anything else) for GetValue, I used the following code:

bool hasPrefix = bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName);
string prefix = ((hasPrefix)&&(bindingContext.ModelName!="")) ? bindingContext.ModelName + "." : "";

If anyone is interested, I ended up inheriting from DefaultModelBinder using something similar to this answer. Here is the full code I used:

public class DefaultPetBinder : DefaultModelBinder
{
    protected override object CreateModel(ControllerContext controllerContext,ModelBindingContext bindingContext,Type modelType)
    {
        //https://stackoverflow.com/questions/5460081/asp-net-mvc-3-defaultmodelbinder-inheritance-problem

        bool hasPrefix = bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName);
        string prefix = ((hasPrefix)&&(bindingContext.ModelName!="")) ? bindingContext.ModelName + "." : "";

        // get the parameter species
        ValueProviderResult result;
        result = bindingContext.ValueProvider.GetValue(prefix+"species");


        if (result.AttemptedValue.Equals("cat"))
            return base.CreateModel(controllerContext,bindingContext,typeof(Cat));
        else if (result.AttemptedValue.Equals("dog"))
            return base.CreateModel(controllerContext,bindingContext,typeof(Dog));
        return base.CreateModel(controllerContext, bindingContext, typeof(Livestock)); // livestock
    }  

}

Answer 1

In the line you’re getting your error, values could be null or what GetValue("name") returns could be null.

I’m assuming when you’re calling the List<Pet> method, ValueProvider is returning the entire List instead of each individual Pet, so it can’t get the value "name" since it doesn’t exist in the List class.

I can’t be more sure without seeing more code.