I am trying to insert data into a 2D array of objects

Question

my code works just fine with 1D array but when i try to make a 2D array i get an error message

the error message :

NullReferenceException: Object reference not set to an instance of an object

this is my code :

public class NewBehaviourScript : MonoBehaviour
{
    
    public CoreManager[] mng2 = new CoreManager[5];
    public CoreManager[][] mng = new CoreManager[5][];
   
    void Start()
    {


        //this code works fine 
        mng2[1].actual_words = new string[5];
        mng2[1].actual_words[0] = "test 1 ";
        Debug.Log(mng2[1].actual_words[0]);

     
        //this code gives me the error message  
        mng[0][0].actual_words = new string[5]; // the error message is about this line  
        mng[0][0].actual_words[0] = "test 2 ";
        Debug.Log(mng[0][0].actual_words[0]);
    }
}

the CoreManager class :

public class CoreManager
{

    [Tooltip("Write these words in actual sequence of the sentence")]
    public string[] actual_words;

    [Tooltip("Write these words in any sequence of the sentence")]
    public string[] mixed_words;

}

Answer 1

What your trying to build here is a Jagged Array . For each additional dimensions of the jagged array, you also need to create an instance of it. In your case, it would look like:

mng[0] = new CoreManager[number of elements you want];
mng[0][0] = new CoreManager();
mng[0][0].actual_words = new string[5]; // the error message is about this line  
mng[0][0].actual_words[0] = "test 2 ";
Debug.Log(mng[0][0].actual_words[0]);

Here's the Microsoft docs on Jagged Arrays.