Getting 'Null Reference' error while working with Array in Unity

[Kurt-Dekker]

The answer is always the same… ALWAYS!

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

Beware of a lot of conflicting ideas embodied in playerInventory: declaring something public, serialized AND initializing it. Those things are never going to play well with each other. Decide first what you are doing rather than applying concepts from many different tutorials you may have seen.

Here’s more about that:

Serialized / public fields in Unity are initialized as a cascade of possible values, each subsequent value (if present) overwriting the previous value:

• what the class constructor makes (either default(T) or else field initializers, eg “what’s in your code”)

• what may be saved with the prefab

• what may be saved with the prefab override(s)/variant(s)

• what may be saved in the scene and not applied to the prefab

• what may be changed in the scene and not yet saved to disk

• what may be changed in OnEnable(), Awake(), Start(), or even later

Make sure you only initialize things at ONE of the above levels, or if necessary, at levels that you specifically understand in your use case. Otherwise errors will seem very mysterious.

Here’s the official discussion: Serialization in Unity

If you must initialize fields, then do so in the void Reset() method, which ONLY runs in the UnityEditor.

Field initializers versus using Reset() function and Unity serialization:

Sensitivity in my MouseLook script.

Whatever value you set and save in your scene or prefab will wipe out the field initializer value you change in the code above. This is a common source of confusion: the object is created, so at that instant in time it has the value you set in code, but before the object is put into use, if Unity has a saved scene or saved prefab that this is already in, then the value saved is overwritten. It's best to remove the initialization value above and instead provide it in a Reset function: publi…

Crouch speed is faster than Movement Speed

If something is public: The instant you add that to your scene / prefab it is 10f … AND THEN IT GETS SERIALIZED. Now go change the value in code. Note that it has no effect: the serialized value writes over the code value. This is how Unity serialization works. If you want to define it in code, make the variable private. If you want to define it in the editor, make it public and DO NOT INITIALIZE IT. Instead, make a Reset function, which ONLY runs at the instant it is added: void Reset(…

To avoid complexity in your prefabs / scenes, I recommend NEVER using the FormerlySerializedAsAttribute

This may also be useful to you as you tackle an inventory system:

These things (inventory, shop systems, character customization, dialog tree systems, crafting, ability unlock systems, tech trees, etc) are fairly tricky hairy beasts, definitely deep in advanced coding territory.

The following applies to ALL types of code listed above, but for simplicity I will call it “inventory.”

Inventory code never lives “all by itself.” All inventory code is EXTREMELY tightly bound to prefabs and/or assets used to display and present and control the inventory. Problems and solutions must consider both code and assets as well as scene / prefab setup and connectivity.

Inventories / shop systems / character selectors all contain elements of:

• a database of items that you may possibly possess / equip
• a database of the items that you actually possess / equip currently
• perhaps another database of your “storage” area at home base?
• persistence of this information to storage between game runs
• presentation of the inventory to the user (may have to scale and grow, overlay parts, clothing, etc)
• interaction with items in the inventory or on the character or in the home base storage area
• interaction with the world to get items in and out
• dependence on asset definition (images, etc.) for presentation

Just the design choices of such a system can have a lot of complicating confounding issues, such as:

• can you have multiple items? Is there a limit?
• if there is an item limit, what is it? Total count? Weight? Size? Something else?
• are those items shown individually or do they stack?
• are coins / gems stacked but other stuff isn’t stacked?
• do items have detailed data shown (durability, rarity, damage, etc.)?
• can users combine items to make new items? How? Limits? Results? Messages of success/failure?
• can users substantially modify items with other things like spells, gems, sockets, etc.?
• does a worn-out item (shovel) become something else (like a stick) when the item wears out fully?
• etc.

Your best bet is probably to write down exactly what you want feature-wise. It may be useful to get very familiar with an existing game so you have an actual example of each feature in action.

Once you have decided a baseline design, fully work through two or three different inventory tutorials on Youtube, perhaps even for the game example you have chosen above.

Breaking down a large problem such as inventory:

Weapon Inventory and how to script weapons.

You’re welcome of course. One thing you could try to really get a “feel” for what happens in this case is make a simple script that is just one cell of your inventory. Put this into a script that you can access through an “inventory API” that does certain things to a cell. These functions might be: “Set the type of item” “Query the type and quantity of item in this cell” “Add 1 item to this cell.” “Add N number of items to this cell.” “Remove N item from this cell.” Then you can have pr…

If you want to see most of the steps involved, make a “micro inventory” in your game, something whereby the player can have (or not have) a single item, and display that item in the UI, and let the user select that item and do things with it (take, drop, use, wear, eat, sell, buy, etc.).

Everything you learn doing that “micro inventory” of one item will apply when you have any larger more complex inventory, and it will give you a feel for what you are dealing with.

Breaking down large problems in general:

Options for implementing mining/digging in an asteroid belt game?

Whenever engineering seems too much to tackle, it's always best to see if you can break the problem down. What you describe above is by no means simple, as just the different ways all of it could be implemented are complex: does the camera rotate with the player on the asteroid? Do the player's controls rotate as they walk around the asteroid? Does the camera lock to the rotating asteroid? What does the camera do if the player digs down a ways? If the player digs a tunnel into the asteroid,…

The moment you put an inventory system into place is also a fantastic time to consider your data lifetime and persistence. Create a load/save game and put the inventory data store into that load/save data area and begin loading/saving the game state every time you run / stop the game. Doing this early in the development cycle will make things much easier later on.