NullReferenceException in MVC3 Controller

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

Actual problem has nothing to do with MVC. You are instantiating ICollection Hazards with anonymous object, and I believe compiler does not resolve this code properly so that it fails at runtime. Here is the quick fix:

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" }
};

Note usage of List<Hazards>{h1, h2} instead of just {h1, h2}.

Answer 2

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>();
    }
    //..
}

Answer 3

Since you're coming from PHP, perhaps it would be more helpful to give you an overview of CSharp's syntax shortcuts!
They're really confusing at first glance because the syntax is SO similar. But after a quick intro, you'll learn to love them.

Collection Initializers

A Collection Initializer adds items to a collection. The syntax is new SOMECOLLECTION { value, value, value }

The traditional way to populate a collection:

var items = new List<int>();
items.Add(1);
items.Add(2);
items.Add(3);

The Collection Initializer way - this compiles to nearly identical code:

var items = new List<int>(){ 1, 2, 3 };

Side note: Array vs List - I'd say List is more commonly used because its size can change. But it's mostly personal preference. The short-hand way to create an array is: var items = new int[]{ 1, 2, 3 };, but you can also omit the type and the compiler will figure it out (aka implicitly-typed array): var items = new []{ 1, 2, 3 };

Object Initializers

An Object Initializer sets the properties of a new object. The syntax is new SOMEOBJECT { Property = value, Property = value }

The traditional way to populate the properties of an object:

var item = new Item();
item.First = "Chuck";
item.Last = "Norris";

The Object Initializer way:

var item = new Item(){ First = "Chuck", Last = "Norris" };

Anonymous Objects

I'm only mentioning this because the syntax for an anonymous object is extremely similar to the others, but does something much, much different. They are extremely confusing if you aren't aware of them!

The syntax looks like a combination of the Collection and Object Initializers: new { AnyPropertyName = value, anyVariableNameBecomesAProperty, someOtherObject.SomeProperty }

Let me explain! The compiler will create an "anonymous object" that has 3 properties, AnyPropertyName, anyVariableNameBecomesAProperty, and SomeProperty, and initializes the values the same way as the Object Initializer does. If you don't specify a property name, the name is inferred from the source.

I hope this helps you understand why new { h1, h2 } (new anonymous object) is WAY different from new []{ h1, h2 } (new array).