Why do I get NullReferenceException here?

Question

I have:

partial class StarSystem : AstroThreeNode
{
    public static StarSystem SunSolarSystem()
    {
        return new StarSystem("Solar System", Planet.SunCenter(), Resources.Space, SolarSystem.ActiveForm.Bounds).AddPlanet(Planet.MercuryPlanet(), Planet.VenusPlanet(), Planet.EarthPlanet(), Planet.MarsPlanet(), Planet.JupiterPlanet(), Planet.SaturnPlanet(), Planet.UranusPlanet(), Planet.NeptunePlanet());
    }

    public StarSystem AddPlanet(params Planet[] planetsToAdd)
    {
        foreach (Planet planet in planetsToAdd)
        {
            distanceFromSun += (planets.Count > 0 ? planets[planets.Count - 1].Image.Width / 2 : 0) + planet.Image.Width / 2 + DISTANCE_BETWEEN_PLANETS;
            planet.DistanceFromSun = distanceFromSun;
            planet.RotationCenter = new PointF(bounds.Width / 2, bounds.Height / 2);
            planets.Add(planet);
        }
        return this;
    }
}


partial class Planet : AstroThreeNode
{
    private const float SUN_SPEED = 0;
    public static Planet SunCenter() 
    {
        return new Planet("Слънце", Resources.Sun_, SUN_SPEED, CLOCKWISE);
    }
    public static Planet MercuryPlanet()
    {
        return new Planet("Меркурий", Resources.Mercury_, 4.0923f, Planet.CLOCKWISE);
    }
    public static Planet VenusPlanet()
    {
        return new Planet("Венера", Resources.Venus_, 1.6021f, Planet.COUNTERCLOCKWISE);
    }
    public static Planet EarthPlanet()
    {
        return new Planet("Земя", Resources.Earth_, 0.9856f, Planet.CLOCKWISE);
    }
    public static Planet MarsPlanet()
    {
        return new Planet("Марс", Resources.Mars_, 0.5240f, Planet.CLOCKWISE);
    }
    public static Planet JupiterPlanet()
    {
        return new Planet("Юпитер", Resources.Jupiter_, 0.0830f, Planet.CLOCKWISE);
    }
    public static Planet SaturnPlanet()
    {
        return new Planet("Сатурн", Resources.Saturn_, 0.0334f, Planet.CLOCKWISE);
    }
    public static Planet UranusPlanet()
    {
        return new Planet("Уран", Resources.Uranus_, 0.0117f, Planet.COUNTERCLOCKWISE);
    }
    public static Planet NeptunePlanet() 
    {
        return new Planet("Нептун", Resources.Neptune_, 0.0059f, Planet.CLOCKWISE);
    }

    public Planet(string name, Image image, float distanceFromSun, float degreesAddedEachTick, bool clockwise, PointF rotationCenter, float angleInDegrees = 0)
    {
        this.Name = name;
        this.Image = image; // here I set the Image but I get an exception later
        this.DistanceFromSun = distanceFromSun;
        this.degreesAddedEachTick = degreesAddedEachTick;
        this.isClockwiseRotation = clockwise;
        this.RotationCenter = rotationCenter;
        this.angleInDegrees = angleInDegrees;
    }
}

I get the exception here: distanceFromSun += (planets.Count > 0 ? planets[planets.Count - 1].Image.Width / 2 : 0) + planet.Image.Width / 2 + DISTANCE_BETWEEN_PLANETS; For the planet.Image. I don't get it why do I get exception when I set the Image when I create a Planet.

EDIT: Here is the constructor for StarSystem :

    public StarSystem(string name, Planet starSystemCenter, Image background, Rectangle bounds)
    {
        this.Name = name;
        this.Background = background;
        this.bounds = bounds;
        planets = new List<Planet>(); //planets instanieted here
        planets.Add(starSystemCenter); // note I dont get the exception on this call but on the other call where I add all other planest
    }

EDIT 2: Found my problem but I have to go to bed I will post the answer tomorrow.

possible duplicate of [What is a NullReferenceException in .NET?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net) — John Saunders, Jan 03 '12 at 21:30
@JohnSaunders I would highly doubt that since almost all nullreferences are different. — Laurence Burke, Jan 03 '12 at 21:32
@John Saunders There is a difference between What is a _____ exception and Why am I getting _____ exception. You can understand what an exception is for but not understand or know what part of your code is the cause of it. — Josh Mein, Jan 03 '12 at 21:34
If you set a breakpoint after the line that sets Planet.Image, is Image null? — sq33G, Jan 03 '12 at 21:35
@LaurenceBurke: Quite the reverse: every NullReferenceException has the same cause, and the steps taken to diagnose it are generally the same too. — Jon Skeet, Jan 03 '12 at 21:36
@JonSkeet so yes they will be similar but not duplicate. Therefore different — Laurence Burke, Jan 03 '12 at 21:37
@jmein: did you read the linked question and its answers? It's about both the causes and the fixes. — John Saunders, Jan 03 '12 at 21:40
@LaurenceBurke: sorry, I totally disagree. Every case of, "I didn't set it, then I used it" is the exact same mistake. The linked question shows many different examples of this, to make it easier to recognize them by sight. — John Saunders, Jan 03 '12 at 21:41
Does the other side of the Planet class have a constructor that takes only 4 arguments? — sq33G, Jan 03 '12 at 21:46
@LaurenceBurke: I don't buy that - it's sufficiently similar, that if the OP reads through the linked question, they're likely to be able to work out their own answer. — Jon Skeet, Jan 03 '12 at 21:49
@sq33G Yes it has `public Planet(string name, Image image, float degreesAddedEachTick, bool clockwise){...}` — Bosak, Jan 03 '12 at 21:56
@sq33G Yes it got hit but I got surprised when I set the `Image` the `image` parameter was not null but when the `this.Image = Image` got ex....Wait a sec..FOUND MY PROB!! I am doing `this.Image = Image` and not `this.Image = image` — Bosak, Jan 03 '12 at 22:07

AllenG · Answer 1 · 2012-01-03T21:58:09.360

2

It looks like you're never instantiating your 'Planets' object (probably a List<Planet>?)

Is it in your StarSystem code?

Presumably it would be something like

public class StarSystem
{
    public List<Planet> planets {get; set;}
    //Other code here

    public StarSystem()
    {
        planets = new List<Planet>();
    }
}

But that code is not in evidence in your post. That's what I find to be the most likely culprit.

Post Question Edit: Okay, then the only thing that seems to leave is the .Image for one or more of your planets. When you debug, you should be able to watch your variables- I would set a break-point before it evaluates that expression, and look at the last object it added, and the next object it's going to try to add, and look for a null .Image

edited Jan 03 '12 at 21:58

answered Jan 03 '12 at 21:32

AllenG

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I am instancing everything I don't understand what are you talking about – Bosak Jan 03 '12 at 21:36
@Bosak - Where is your line where `Planets` is instantiated? `var Planets = ???` – AllenG Jan 03 '12 at 21:38
1

He's talking about `planets.Count`. He doesn't think `planets` was initialized. – Daniel Moses Jan 03 '12 at 21:38
@DMoses: that's what I think, too. – AllenG Jan 03 '12 at 21:42
But I'm sure that `planets` is initalised – Bosak Jan 03 '12 at 21:46
@Bosak - How? There's nothing in your code that references 'planets' until the line `planets.Add(planet);` – AllenG Jan 03 '12 at 21:47
@AllenG I edited the question and added the `StarSystem` constructor – Bosak Jan 03 '12 at 21:50
I except that the problem may be that the planet creator methods are static and something is not getting executed there. Maybe its because `Image` has a default value of `null`, but I still cant find where is the error – Bosak Jan 03 '12 at 22:04

score 1 · Accepted Answer · answered Jan 03 '12 at 22:35

1

Try to have class members, local variables, and method parameters differ by more than capitalization. See Bad Naming Convention in the canonical NullReferenceException post linked above.

answered Jan 03 '12 at 22:35

sq33G

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score 0 · Answer 3 · answered Jan 03 '12 at 21:35

0

You create each planet with this constructor

public Planet(string name, Image image, float distanceFromSun, float degreesAddedEachTick, bool clockwise, PointF rotationCenter, float angleInDegrees = 0)

Yet you never check to make sure that image != null && image.Width != null (if width is a native int you don't need to, but if it is a Number you do need that check.

answered Jan 03 '12 at 21:35

Daniel Moses

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It is a native int. I get the exception for the image not for the with . – Bosak Jan 03 '12 at 21:38
Then i suggest putting a `if (image == null) throw new IllegalArgumentException();` in the Planet constructor. See if that gives you a different error. Should be able to tell who is setting a null image. – Daniel Moses Jan 03 '12 at 21:41
No it gives the same exception – Bosak Jan 03 '12 at 21:58
Then you are doing 1 of two things: not actually using that costructor... i noticed you use a constructor with 4 parameters? Or you change the image member variable to null after it is instantiated. – Daniel Moses Jan 03 '12 at 22:11

Why do I get NullReferenceException here?

3 Answers3