Array NullReferenceException

Question

So, i didnt really find anything that helped me in other threads, so here is myproblem: I want to generate random Quests for my game so i created one script that is not on an object containing the following code:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
     
 [System.Serializable]
 
 public class QuestsType : System.Object
 {
     public string questName;
     public string giverName;
     public int type;
     public int difficulty;
     public int questLength;
 }

and then i have this second script, wich i hoped would give me the value of the variable "type" of each of the generated arrays (just for testing):

     public int questAmountMin;
     public int questAmountMax;

     public QuestsType[] quests;
 
     void Start ()
     {
         GenerateQuests();
     }
 
     void GenerateQuests()
     {
         quests = new QuestsType[Random.Range(questAmountMin,questAmountMax)];
         for (int i = 0;  i < quests.Length ; i++)
         {
             print(quests[i].type);
         }
     }

It will then always (except if the random "quests" array has a length of 0) give me a NullReferenceException: Object reference not set to an instance of an object - error. The strange thing now is that it acually works, but only if i manually execute the

         for (int i = 0;  i < quests.Length ; i++)
         {
             print(quests[i].type);
         }

-portion via a button or something, and ONLY IF the inspector shows the Gameobject with the second script. ive never seen something like that before..

I would really appreciate your help!

Answer 1

Best Answer

Answer by Pharan · Oct 04, 2015 at 11:40 AM

Instantiating the array is a different step from instantiating the objects inside the array.

Because QuestsType is a class, the array QuestsType[] quests is an array of references to QuestsType. And all references are initialized by default as null references (references that point to nothing) until you create something it can point to.

When you first create the array (line 13), it's an array of references to nothing.

You need to go:


    for (int i = 0; i < quests.Length; i++)
    {
         quests[i] = new QuestsType();
         // then you can do this
         print(quests[i].type);
    }

There's also no need for QuestsType to be declared as QuestsType : System.Object. All declared classes already inherit from System.Object without you writing anything on there.

What programming language did you come from? So we can point you in more helpful directions.

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Tronnyi · Oct 04, 2015 at 11:50 AM 0

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Thank you very much! That seems to work. I am learning it by myself and are still very much a scrub, using the internet and those forums a lot, also im co$$anonymous$$g from JS.

Pharan Tronnyi · Oct 04, 2015 at 03:01 PM 0

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Glad to help. It's cool that you're jumping straight into C#. C# code samples and documentation are easier to find.

Unity's managed/scripting runtime is $$anonymous$$ono/.NET. It's just one generation ahead of Java, and takes most its paradigms from it (class-based OOP that's way different from javascript's prototype-centric OOP, abstraction of pointer operations, single-inheritance, managed memory/garbage collection, exceptions) but improves on it with the aim of cleaner code in C# (value-type structs, get-set properties and auto-properties, events, delegates and lambdas, extension methods). Anyway, point is that you'll find more weird stuff if you come from a javascript perspective.

The basic topic you ran into was "the difference between reference types and value types". You can look that up if you want to get a solid grasp of it.

Array NullReferenceException

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