C# Object Reference not set to an instance of an object error

Question

I am getting the error in the title when I run this code during the Add method. The Add method should add the gameobject to a list called queue.
GameObject is a class.
GameManager is a class as well.
queue is a list.

I think this is the only code relevant.

    static void Main()
    {
        GameObject obj1 = new GameObject();
        GameManager manager1 = new GameManager();
        obj1.name = "First";
        manager1.Add(obj1);
        manager1.Process();
    }

    public void Add(GameObject gameObject)
    {
        gameObject.initialize = true;
        queue.Add(gameObject);
    }

Did you ever initialize the list? Also, there is a `Queue` that may be more appropriate than a `List`. — Mike Precup, Jul 09 '13 at 19:59
Almost all cases of `NullReferenceException` are the same. Please see "[What is a NullReferenceException in .NET?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net)" for some hints. — John Saunders, May 15 '14 at 19:20

Ron.B.I · Accepted Answer · 2013-07-09T20:20:51.567

3

Try initializing the field "queue":

It's probably never initialized, hence the Object Reference not set to an instance of an object error error, which by the structure of the Add method is likely to correspond to queue.
Also - I would consider renaming queue to a meaningful name, since it is currently deceiving.

Here is an initialization suggestion, it is for a List as you described in your question, but can easily be modified to correspond to any IEnumerable , also, it can be done in a different method being executed prior to Main (again, by the looks of your code it seems unlikely) :

private List<GameObject> queue; // assuming it's private, doesn't really matter either way.

static void Main()
{
    queue = new List<GameObject>(); // the missing line
    GameObject obj1 = new GameObject();
    GameManager manager1 = new GameManager();
    obj1.name = "First";
    manager1.Add(obj1);
    manager1.Process();
}


public void Add(GameObject gameObject)
{
    gameObject.initialize = true;
    queue.Add(gameObject);
}

edited Jul 09 '13 at 20:20

answered Jul 09 '13 at 20:02

Ron.B.I

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How do you know queue is a List could be a Dictionary or anothing? – Bit Jul 09 '13 at 20:05
Have you considered that the queue is in fact a System.Collections.Generics.Queue<>? – Pharap Jul 09 '13 at 20:12
1

It's in the OP description - "queue is a list", either way , I will add clarification since it doesn't really matter what type of IEnumerable it is, all need initialization :) – Ron.B.I Jul 09 '13 at 20:16
Thanks that fixed it. Should have looked more into lists. – M K Jul 09 '13 at 20:26
1

So he did sorry @Ron B. – Bit Jul 09 '13 at 20:56

score 1 · Answer 2 · answered Jul 09 '13 at 20:02

1

You have to initialize Queue list first

answered Jul 09 '13 at 20:02

Eslam Gamal

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5

score 0 · Answer 3 · answered Jul 09 '13 at 20:04

0

You need to debug your code.

I assume you are using Visual Studio, if so then do this:

Go to the Debug menu.
Click on the the Exceptions... choice.

The following dialog should appear:

enter image description here

Note: the Common Language Runtime Exceptions checkbox is checked.

Upon clicking OK, now when you debug your code anytime an exception is thrown by your code or the .NET Framework, the debugger will halt on the line that threw the exception. This makes finding where something is "breaking" much easier.

answered Jul 09 '13 at 20:04

Karl Anderson

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This would only really help for exceptions which are caught/handled/swallowed. For an unhandled exception like the one the OP described, the debugger is going to break anyway. – MattDavey Jul 10 '13 at 08:30
@MattDavey - No, this will break as soon as an exception is thrown; not when it bubbles all the way up the chain. – Karl Anderson Jul 10 '13 at 11:45
I understand that, but for unhandled exceptions you're not going to see any difference in behaviour by doing this. The debugger is still going to break on the line that threw the exception regardless. – MattDavey Jul 10 '13 at 12:08

C# Object Reference not set to an instance of an object error

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