error in c# for creating list of quiz data

Questions & Answers
1

Dear all,
I am storing json quiz data into list using C# but I get error. Here is json file and two c# scripts:

Json file:

{
"data":[
{
	"id":"1",
	"Q":"Mechanics deals with the study of",
	"Ans":[
		"motion of bodies",
		"origin of universe",
		"machines",
		"heat energy"
	]
	
},
{
	"id":"2",
	"Q":"Electrons were discovered by",
	"Ans":[
		"JJ Thomson",
		"Rutherford",
		"Max Plank",
		"Einstein"
	]
},
{
	"id":"3",
	"Q":"The SI unit of electric current is",
	"Ans":[
		"Ampere",
		"Volt",
		"Watt",
		"KWH"
	]
},
{
	"id":"4",
	"Q":"which one is not base quantity?",
	"Ans":[
		"force",
		"mass",
		"time",
		"distance"
	]
},
{
	"id":"5",
	"Q":"which of the following is not a unit of time?",
	"Ans":[
		"Light Year",
		"Year",
		"Second",
		"microsecond"
	]
},
{
	"id":"6",
	"Q":"Newton-sec is unit of",
	"Ans":[
		"linear momentum",
		"force",
		"work",
		"power"
	]
},
{
	"id":"7",
	"Q":"Era of modern physics begins from:",
	"Ans":[
		"1900",
		"1800",
		"1924",
		"1979"
	]
},
{
	"id":"8",
	"Q":"Special Theory of relativity was given out by Einstein in",
	"Ans":[
		"1905",
		"1916",
		"1901",
		"1924"
	]
},
{
	"id":"9",
	"Q":"Einstein received Noble Prize on his work on:",
	"Ans":[
		"Photoelectric effetc",
		"Special Theory of Relativity",
		"General Theory of Relativity",
		"E=mc²"
	]
}
]
}

c# class :

using System.Collections.Generic;

public class QuizData
{
	public string id { get; set;}
	public string Q { get; set;}
	public List<string> Ans { get; set;}
}

//C# file which handles data:

using UnityEngine;

using System.Collections;

using System.IO;

using LitJson;	
using UnityEngine.UI;

using System.Collections.Generic;

public class QuizBank : MonoBehaviour {

	public string jsonString;
	public JsonData qData;
	public int q_counter=1;
	public int quizTotal;

	public QuizData quiz = new QuizData();
	public List<QuizData> quizList = new List<QuizData> ();

	public void JData () {
		string filePath = System.IO.Path.Combine (Application.streamingAssetsPath, "Test 1.json");
		jsonString = System.IO.File.ReadAllText (filePath);
		qData = JsonMapper.ToObject (jsonString);
		for (int i = 0; i < 5; i++) {
			quiz.Q= qData ["data"] [q_counter] ["Q"].ToString ();
			for (int j = 0; j < 4; j++) {
				quiz.Ans.add((qData ["data"] [q_counter] ["Ans"] [j].ToString ()));
			}
			
			quizList.Add(quiz);
			
			q_counter++;
			
		}
		
	}
}

But I get error “Object reference not set to an instance of an object” at the line where i add “quiz.Ans.add”.
Any help would be appreciated.
Thank you,
Ram

2

You need to instantiate the quiz.Ans List just as you do it with the quizList. That means you need to quiz.Ans = new List<string>(); somwhere, otherwise it is never being instantiated and therefore is null. EDIT: I would recommend you to do this in the QuizData class constructor:

public class QuizData
 {
     public string id { get; set;}
     public string Q { get; set;}
     public List<string> Ans { get; set;}
     public QuizData()
     {
          Ans = new List<string>();
     }

 }

EDIT: as of your 2nd question I’m not sure if I understand you correctly.

But what you probably want is a list of all QuizData objects. Did you try the following:

public void JData () {
         string filePath = System.IO.Path.Combine (Application.streamingAssetsPath, "Test 1.json");
         jsonString = System.IO.File.ReadAllText (filePath);
         quizList = JsonMapper.ToObject<QuizDataRoot>(jsonString);

}

Where QuizDataRoot is:

public class QuizDataRoot {
    public List<QuizData> data { get; set; }
}

And then for debugging:

void Start() {
         JData ();
         Debug.Log (quizList [2].Q); //print question number 3
         foreach(QuizData quizData in quizList) {
               Debug.Log(quizData.id+", "+quizData.Q);
               foreach(string a in quizData.Ans) {
                    Debug.Log(a);
               }
         }
}

Hope this helps

3

Dear InfiniBuzz!!

Thank you very much, a lot of thanks…