Question

I’m playing around with MVC3, trying to create small test projects which simulate problems that I anticipate I will encounter in a larger project I’m migrating from PHP. I keep getting a “NullReferenceException” on instantiating the “model” variable in the project with the elements below:

Model:

public class MainModel
{
    public IEnumerable<Chemical> chemicals { get; set; }

}

public class Chemical
{
    public int ChemicalId { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Hazard> Hazards { get; set; }

}

public class Hazard
{
    public int HazardId { get; set; }
    public string Description { get; set; }

    public virtual ICollection<Chemical> Chemicals { get; set; }
}

Controller:

public class MainController : Controller
{

    public ActionResult Index()
    {

        var h1 = new Hazard { HazardId = 1, Description = "Flammable" };
        var h2 = new Hazard { HazardId = 2, Description = "Carcinogen" };
        var h3 = new Hazard { HazardId = 3, Description = "Water Reactive" };

        var model = new [] {
            new Chemical { ChemicalId = 1, Name = "Benzene", Hazards = {h1, h2}},
            new Chemical { ChemicalId = 2, Name = "Sodium", Hazards = { h3 } },
            new Chemical { ChemicalId = 3, Name = "Chloroform", Hazards =  { h2 } },
            new Chemical { ChemicalId = 4, Name = "Water" }
        }; //NULL EXCEPTION THROWN HERE

        return View(model);
    }

}

So, I have two questions:

1. Why do I keep getting the NullReferenceException, and how could it be fixed?
2. In writing this code, I used this StackOverFlow Page as a starting point. Why is this okay, but what I wrote is not? Granted, I didn’t test this code, but the author seems very competent.

Thanks!

Answer 1

Why do I keep getting the NullReferenceException, and how could it be
fixed?

The ICollection property Hazards is null initially – you have to use a constructor when you create your model:

var model = new[] {
    new Chemical { ChemicalId = 1, Name = "Benzene", Hazards = new List<Hazard>() {h1, h2}},
    new Chemical { ChemicalId = 2, Name = "Sodium", Hazards = new List<Hazard>(){ h3 } },
    new Chemical { ChemicalId = 3, Name = "Chloroform", Hazards =  new List<Hazard>(){ h2 } },
    new Chemical { ChemicalId = 4, Name = "Water" }
};

The code you currently have will try to add i.e. {h1,h2} to Hazards – which is null and throw a NullReferenceException. An alternative is to create a new empty Hazards collection in the Chemical constructor – then you can leave your current code unchanged:

public class Chemical
{
    public Chemical()
    {
        _hazards = new List<Hazard>();
    }
    //..
}