Why does this code have such a result?

Question

C++ / C#

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class test : MonoBehaviour {

  public Save sv;

  public void Awake()
  {
    sv = JsonUtility.FromJson<Save>(PlayerPrefs.GetString("SV"));
    Debug.Log(sv.test.Length);
    Debug.Log("Begin - " + sv.achieveStatus.Length);		
  }
  public void OnApplicationQuit()
  {		
    Debug.Log(sv.test.Length);
    Debug.Log("End - " + sv.achieveStatus.Length);
    PlayerPrefs.SetString("SV", JsonUtility.ToJson(sv));
    PlayerPrefs.Save();
  }
}

[System.Serializable]
public class Save
{
  public byte[] test;
  public byte[,] achieveStatus;
}

this code throws an error: object reference not set to an instance of an object
After that, I initialized the arrays and it turned out like this:

public byte[] test = new byte[3];
public byte[,] achieveStatus = new byte[3,3];

On startup and on exit, the code outputs 3 and 9 to the console. But then, when I removed new byte[3] and new byte[3,3] . And I ran 3 in the console and object reference not set to an instance of an object and the same thing happens when I exit. That is, in the player prefs, the one-dimensional array is preserved, but the two-dimensional one, it turns out that it is not. Why is that?

Answer 1

not "not saved".
and the Json serializer does not know how to work with multidimensional arrays.
replace it with something in the spirit of .. an array of arrays or a list of lists))
this can be serialized, and access to the same indexes will remain.

Answer 2

For JSON in C#, there is an excellent class generation service and library: Newtonsoft.Json