Question reference becomes null when passed into method

I have some code where a script is supposed to add itself to an array in a singleton, like so:

Initialize() in Structure

Code (CSharp):

1. public void Initialize()
2.     {
3.         gameObject.layer = LayerMask.NameToLayer("Structure");
4.         if(this == null)
5.         {
6.             UnityEngine.Debug.Log("this is null");
7.         }
8.         GridMgr.gridMgr.Add(this);
9.     }

Add() in GridMgr

Code (CSharp):

1. public void Add(Structure structure)
2.     {
3.         if (structures == null)
4.         {
5.             UnityEngine.Debug.Log("structures is null");
6.         }
7.         structures.Add(structure);
8.     }

The issue is that "this" is not null before it is passed into Add(), but Structure is null within Add(). Why is this value being nulled? I am not messing with the values in any other scripts that might be running concurrently, although I don't think that matters because this shouldn't be multithreaded anyways.

 

nevermind

 

S'all guud sir... lists need to be initialized too. That's why I have my three step checklist.

In your case you hadn't actually found the thing that was null... until you did.

How to fix a NullReferenceException error

https://forum.unity.com/threads/how-to-fix-a-nullreferenceexception-error.1230297/

Three steps to success:
- Identify what is null <-- any other action taken before this step is WASTED TIME
- Identify why it is null
- Fix that

That's why I patiently tell folks who insist that something isn't null:

"That's great, it just means you haven't found the thing that IS null. Keep looking!"

 

Just in case someone comes across this thread:
This

structures

is not the same as

structure

So make sure you actually use the correct variable. I personally usually prefix "arguments" of methods with an "a" (for argument). This avoids name collisions with member variables and also makes it easier to recognise it as an argument. However different people, different styles.

Code (CSharp):

1. public void Add(Structure aStructure)
2. {
3.     if (aStructure == null)
4.         UnityEngine.Debug.LogError("passed Structure 'aStructure' is null");
5.     if (structures == null)
6.         UnityEngine.Debug.LogError("'structures' List is null");
7.     structures.Add(structure);
8. }