System Null Reference Exception (card matching game)

I am making a card matching game and i have a System Null Reference Exception which only pops up when I am playing the game and not when I check for errors. I am unsure on how to fix it and would like some help.

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardControllerScript : MonoBehaviour {
6.  
7.     private Sprite CardBack = null;
8.     private bool matched;
9.     public bool check;
10.     public bool faceUp;
11.     private CardGame tempGame;
12.  
13.     void Start () {
14.         matched = false;
15. }
16.         void OnMouseDown (){
17.         if(!faceUp){
18.            
19.         gameObject.SetActive (false);
20.    
21.             CardGame.count++;
22.             if (CardGame.CardBack != CardBack.ToString () && CardGame.count == 1){   //Null Reference Exception is for this line
23.                 CardGame.CardBack = CardBack.ToString();
24.             }else{
25.                 tempGame.CheckWin(CardBack.ToString());
26.         }
27.     }
28. }
29.  
30.     public bool GetMatched(){
31.     return matched;
32. }
33.  
34. public bool GetIsFaceUp(){
35.     return faceUp;
36. }
37.  
38. public void SeMatchedToTrue(){
39.     matched = true;
40. }
41.        
42. public void SetCardSprite(Sprite newSprite){
43.         CardBack = newSprite;
44.     }
45.  
46. }

If someone could help me fix it that would be greatly appreciated.

here is my other script if it helps

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardGame : MonoBehaviour {
6.    
7.     public static string CardBack = null;
8.     public static int count = 0;
9.     private Component[] cards;
10.  
11.     // Use this for initialization
12.     void Start () {
13.         CardBack = "";
14.         cards = this.GetComponentsInChildren<CardGame>();
15.     }
16.  
17.     // Update is called once per frame
18.     void Update () {
19.  
20.     }
21.  
22.     public void CheckWin(string secondCard){
23.  
24.         if (count == 2){
25.             if (CardBack == secondCard){
26.                 Debug.Log ("Matched!");
27.                 count = 0;
28.                 foreach(CardControllerScript card in cards){
29.                     if(card.ToString() == CardBack){
30.                         Debug.Log ("you got 1 point");
31.                         card.SeMatchedToTrue();
32.                     }
33.                 }
34.             }else{
35.                 Debug.Log ("Not match!");
36.                 count = 0;
37.                 foreach(CardControllerScript card in cards){
38.                     if(card.GetMatched() == false){
39.                         gameObject.SetActive (true);
40.                     }
41.                 }
42.             }
43.         } // if count = 2
44.     }
45.  
46. }
47.  
48.  

 

What is the exact error message (including script and line number)?

 

I think the issue might be line 14 in CardGame
cards = this.GetComponentsInChildren<CardGame>();
this script is the parent gameobject, looking for all children with the same script, so if grand child will have 0 children under it and the array will be null.
then you'll get null ref error on line 28 and line 37

if the script CardControllerScript is in each of the children then you could change the variable cards from Component to CardControllerScript
CardControllerScript[] cards;
then change the get to
cards = this.GetComponentsInChildren<CardControllerScript>();

that should stop the null ref

 

line 22 in CardControllerScript is null because you're calling the script not the variable.
change it to tempGame not CardGame

1. tempGame.count++;
2. if (tempGame.CardBack != tempGame.ToString () && tempGame.count == 1){ //Null Reference Exception is for this line
3. tempGame.CardBack = tempGame.ToString();
4. }else{
5. tempGame.CheckWin(tempGame.ToString());
6. }

edit:
reading this again and again
you are creating a variable called tempGame of type CardGame, but in the If statement you where checking if CardGame.CardBack doesnot = tempGame.ToString(), So now i'm thinking that the .ToString() is an issue. you're forcing to be a string when it's not, because it's a CardGame.
can you give me a Debug.Log(tempGame.ToString()); and tell me what is outputs

 

oh man and CardBack is static string that is set to null, then in Start() you set it to ""
My second post might not work with this being a static variable.

set it to "" from the beginning.
change
public static string CardBack = null;

to
public static string CardBack = "";

 

I did all of your suggestions and now I have 4 errors stating that static member 'CardGame.count' cannot be accessed with an instance reference qualify with a type name instead for lines 21-23 with 2 of these for 22. all of the errors are these. I am unsure of how to fix this and I am unsure of how the Debug.log(tempGame.ToString) works and how to check the output. I am new to unity.

 

i changed the lines below back to CardGame to fix the errors but now the System Null Reference Exception is on line 24 or the line that says }else{. How do i fix this.

1. tempGame.count++;
2. if (tempGame.CardBack
   
3. tempGame.count
   
4. tempGame.CardBack

i also reposted the scripts.

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardControllerScript : MonoBehaviour {
6.  
7.     private Sprite CardBack = null;
8.     private bool matched;
9.     public bool check;
10.     public bool faceUp;
11.     private CardGame tempGame;
12.  
13.     void Start () {
14.         matched = false;
15. }
16.         void OnMouseDown (){
17.         if(!gameObject.SetActive (false)){
18.  
19.             CardGame.count++;
20.             if (CardGame.CardBack != tempGame.ToString () && CardGame.count == 1){
21.                 CardGame.CardBack = tempGame.ToString();
22.             }else{ //null reference exception is this line
23.                 tempGame.CheckWin(tempGame.ToString());
24.             }Debug.Log(tempGame.ToString());
25.     }
26. }
27.  
28.     public bool GetMatched(){
29.     return matched;
30. }
31.  
32. public bool GetIsFaceUp(){
33.     return faceUp;
34. }
35.  
36. public void SeMatchedToTrue(){
37.     matched = true;
38. }
39.      
40. public void SetCardSprite(Sprite newSprite){
41.         CardBack = newSprite;
42.     }
43.  
44. }

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardGame : MonoBehaviour {
6.  
7.     public static string CardBack = "";
8.     public static int count = 0;
9.     private Component[] cards;
10.  
11.     // Use this for initialization
12.     void Start () {
13.         CardBack = "";
14.         cards = this.GetComponentsInChildren<CardControllerScript>();
15.     }
16.  
17.     // Update is called once per frame
18.     void Update () {
19.  
20.     }
21.  
22.     public void CheckWin(string secondCard){
23.  
24.         if (count == 2){
25.             if (CardBack == secondCard){
26.                 Debug.Log ("Matched!");
27.                 count = 0;
28.                 foreach(CardControllerScript card in cards){
29.                     if(card.ToString() == CardBack){
30.                         Debug.Log ("you got 1 point");
31.                         card.SeMatchedToTrue();
32.                     }
33.                 }
34.             }else{
35.                 Debug.Log ("Not match!");
36.                 count = 0;
37.                 foreach(CardControllerScript card in cards){
38.                     if(card.GetMatched() == false){
39.                         gameObject.SetActive (true);
40.                     }
41.                 }
42.             }
43.         } // if count = 2
44.     }
45.  
46. }
47.  
48.  

 

looking into this. check back in a bit. i should have an answer for you

 

ok, so the main issue is that you have a variable tempGame but it's null, you have never assigned anything to it.
you created a place to store it when you did "private CardGame tempGame;" but then never put anything in it.

So you'll need to help me. what is this going to used for? you are trying to check it's CardBack against the static CardBack.

 

i've fixed most of it, i'm getting a null ref on CardGame now.

here is my code

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardGame : MonoBehaviour
6. {
7.  
8.     public static string CardBack = "";
9.     public static int count = 0;
10.     private Component[] cards;
11.     public string cardback;
12.  
13.     // Use this for initialization
14.     void Start()
15.     {
16.         CardBack = "null";
17.         cards = this.GetComponentsInChildren<CardControllerScript>();
18.     }
19.  
20.     // Update is called once per frame
21.     void Update()
22.     {
23.  
24.     }
25.  
26.     public void CheckWin(string secondCard)
27.     {
28.  
29.         if (count == 2)
30.         {
31.             if (CardBack == secondCard)
32.             {
33.                 Debug.Log("Matched!");
34.                 count = 0;
35.                 foreach (CardControllerScript card in cards)
36.                 {
37.                     if (card.ToString() == CardBack)
38.                     {
39.                         Debug.Log("you got 1 point");
40.                         card.SeMatchedToTrue();
41.                     }
42.                 }
43.             }
44.             else
45.             {
46.                 Debug.Log("Not match!");
47.                 count = 0;
48.                 foreach (CardControllerScript card in cards)
49.                 {
50.                     if (card.GetMatched() == false)
51.                     {
52.                         gameObject.SetActive(true);
53.                     }
54.                 }
55.             }
56.         } // if count = 2
57.     }
58.  
59. }

and

Code (CSharp):

1.  
2. using System.Collections;
3. using System.Collections.Generic;
4. using UnityEngine;
5.  
6. public class CardControllerScript : MonoBehaviour
7. {
8.  
9.     private Sprite CardBack = null;
10.     private bool matched;
11.     public bool check;
12.     public bool faceUp;
13.     private CardGame tempGame;
14.  
15.     void Start()
16.     {
17.         tempGame = new CardGame(); //creating a new one
18.         tempGame.cardback = gameObject.name; //seting it's local cardback name
19.         matched = false;
20.     }
21.     void OnMouseDown()
22.     {
23.         Debug.Log("Static CardBack : " + CardGame.CardBack);
24.         Debug.Log("tempgame.cardback : " + tempGame.cardback);
25.         CardGame.count++;
26.  
27.         if (CardGame.CardBack != tempGame.cardback && CardGame.count == 1)
28.         {
29.             //not the same
30.             Debug.Log("Not the same");
31.             CardGame.CardBack = tempGame.cardback;
32.         }
33.         else
34.         {
35.             // same, so it's a match
36.             Debug.Log("Match found");
37.             tempGame.CheckWin(tempGame.cardback);
38.         }
39.  
40.          
41.      
42.     }
43.  
44.     public bool GetMatched()
45.     {
46.         return matched;
47.     }
48.  
49.     public bool GetIsFaceUp()
50.     {
51.         return faceUp;
52.     }
53.  
54.     public void SeMatchedToTrue()
55.     {
56.         matched = true;
57.     }
58.  
59.     public void SetCardSprite(Sprite newSprite)
60.     {
61.         CardBack = newSprite;
62.     }
63.  
64. }

 

sorry i missed this line in my script:
Code (CSharp):

1. tempGame = GameObject.Find ("CardGame").GetComponent<CardGame>();

 

I changed it to your code and added the line i missed out and now the system null reference exception is for line 24 which is

Code (CSharp):

1. CardGame.count++;

EDIT: just realised you already said that.

 

i'm still working on it. give me a few more mins

 

sure that is fine take as long as you need. Thanks for all the help by the way.

 

All my error are removed, So I used the Card name to get matches, so like card1, card2, card3. I have 2 of each of them in my scene.
You can remove my Debug, they are just there to help me follow the code.
just a recap, My parent gameobject has the CardGame script attached to it. each card has the CardControllerScript attached to it.
and each card is a subobject of the parent

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardGame : MonoBehaviour
6. {
7.  
8.     public static string CardBack = "null";
9.     public static int count = 0;
10.     private CardControllerScript[] cards;
11.     public string cardback;
12.  
13.     // Use this for initialization
14.     void Start()
15.     {
16.         CardBack = "null";
17.         cards = this.GetComponentsInChildren<CardControllerScript>();
18.         Debug.Log(cards.Length);
19.     }
20.  
21.     // Update is called once per frame
22.     void Update()
23.     {
24.  
25.     }
26.  
27.     public void CheckWin(string secondCard)
28.     {
29.         Debug.Log("CheckWin : " + CardBack + " : " + secondCard);
30.         if (count == 2)
31.         {
32.             if (CardBack == secondCard)
33.             {
34.                 Debug.Log("Matched!");
35.                 count = 0;
36.                 for (int i = 0; i < cards.Length; i++)
37.                 {
38.                  
39.                     if (cards[i].getCardBack() == secondCard)
40.                     {
41.                         Debug.Log("SeMatchedToTrue");
42.                         cards[i].SeMatchedToTrue();
43.                     }
44.                  
45.                 }
46.                 Debug.Log("you got 1 point");
47.             }
48.             else
49.             {
50.                 Debug.Log("Not match!" + cards.Length);
51.                 count = 0;
52.                 for (int i = 0; i < cards.Length; i++)
53.                 {
54.                  
55.                     if (cards[i].GetMatched() == false)
56.                     {
57.                         cards[i].gameObject.SetActive(true);
58.                     }
59.                     CardBack = "null";
60.                 }
61.             }
62.         } // if count = 2
63.     }
64.  
65. }
66.  

and

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardControllerScript : MonoBehaviour
6. {
7.  
8.     private Sprite CardBack = null;
9.     private bool matched;
10.     public bool check;
11.     public bool faceUp;
12.     private CardGame tempGame;
13.  
14.     void Start()
15.     {
16.         //tempGame = new CardGame(); //creating a new one
17.         tempGame = gameObject.GetComponentInParent<CardGame>();
18.         tempGame.cardback = gameObject.name; //seting it's local cardback name
19.         matched = false;
20.     }
21.     void OnMouseDown()
22.     {
23.         Debug.Log("Static CardBack : " + CardGame.CardBack);
24.         Debug.Log("cardback : " + gameObject.name);
25.         CardGame.count++;
26.  
27.         Debug.Log("count = " + CardGame.count);
28.         if (CardGame.count == 1)
29.         {
30.             //not the same
31.             Debug.Log("not 2 picked");
32.             CardGame.CardBack = gameObject.name;
33.         }
34.         else
35.         {
36.             // Check if they are the same
37.             Debug.Log("check if same");
38.             tempGame.CheckWin(gameObject.name);
39.         }
40.  
41.          
42.      
43.     }
44.  
45.     public bool GetMatched()
46.     {
47.         return matched;
48.     }
49.  
50.     public bool GetIsFaceUp()
51.     {
52.         return faceUp;
53.     }
54.  
55.     public void SeMatchedToTrue()
56.     {
57.         matched = true;
58.     }
59.  
60.     public void SetCardSprite(Sprite newSprite)
61.     {
62.         CardBack = newSprite;
63.     }
64.  
65.     public string getCardBack()
66.     {
67.         return gameObject.name;
68.     }
69. }

 

you still have some logic to clean up. swapping out the sprite, enabling or disabling the card when it has been matched. stopping the playing from clicking on the same card

 

so the cards that matched have to be named the same or not

 

yes, they need to be named the same.
or you have to create a public string and check everything based on that string

 

ok thanks, where you would put the enabling and disabling of the cards in the script. i would imagine that the disabling would be after the on mouse down command and the enabling would be in the not match section.

 

so if the card was two cards named cardback that would work and two cards named cardback(1) and so on

 

yes, that would work.

or via a string like cardIdName

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardControllerScript : MonoBehaviour
6. {
7.  
8.     private Sprite CardBack = null;
9.     private bool matched;
10.     public bool check;
11.     public bool faceUp;
12.     private CardGame tempGame;
13.     public string cardIdName;
14.  
15.     void Start()
16.     {
17.         if(cardIdName == "" || cardIdName == null)
18.         {
19.             Debug.Log("Set the name in Unity, on each card");
20.             return;
21.         }
22.         tempGame = gameObject.GetComponentInParent<CardGame>();
23.         matched = false;
24.     }
25.     void OnMouseDown()
26.     {
27.         Debug.Log("Static CardBack : " + CardGame.CardBack);
28.         Debug.Log("cardback : " + cardIdName);
29.         CardGame.count++;
30.  
31.         Debug.Log("count = " + CardGame.count);
32.         if (CardGame.count == 1)
33.         {
34.             //not the same
35.             Debug.Log("not 2 picked");
36.             CardGame.CardBack = cardIdName;
37.         }
38.         else
39.         {
40.             // Check if they are the same
41.             Debug.Log("check if same");
42.             tempGame.CheckWin(cardIdName);
43.         }
44.  
45.          
46.      
47.     }
48.  
49.     public bool GetMatched()
50.     {
51.         return matched;
52.     }
53.  
54.     public bool GetIsFaceUp()
55.     {
56.         return faceUp;
57.     }
58.  
59.     public void SeMatchedToTrue()
60.     {
61.         matched = true;
62.     }
63.  
64.     public void SetCardSprite(Sprite newSprite)
65.     {
66.         CardBack = newSprite;
67.     }
68.  
69.     public string getCardBack()
70.     {
71.         return cardIdName;
72.     }
73. }

 

line 20

line 20 which says

Code (CSharp):

1. return;

is a system null reference exception
EDIT: also how would you stop the player from selecting the same card twice if it is deactivated could they do that

 

you're getting a null ref from line 20, no way

 

yeah how does that happen and how do i fix it

 

this why you're learning how to program. programming is all about answering questions. there are many ways to do something.

 

just remove the return; line. it's not needed

 

ok thanks that worked

 

line 35
Code (CSharp):

1. CardGame.CardBack = cardIdName;

should change the thread name to system null reference exceptions for probably every line at this point but yeah this is also a null reference exceptions

EDIT: sorry for taking all of your time i really appreciate what you are doing for me and also this isn't a null reference exception anymore not sure why it was before and not now didn't change anything

 

every time i run to cursor on mono develop it tells me something different is wrong and i am unsure if it is a bug or this is normal

 

please tell me you copied all my code.

 

yeah why

 

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardControllerScript : MonoBehaviour
6. {
7.  
8.     private Sprite CardBack = null;
9.     private bool matched;
10.     public bool check;
11.     public bool faceUp;
12.     private CardGame tempGame;
13.     public string cardIdName;
14.  
15.     void Start()
16.     {
17.         if(cardIdName == "" || cardIdName == null)
18.         {
19.             Debug.Log("Set the name in Unity, on each card");
20.             }
21.         tempGame = gameObject.GetComponentInParent<CardGame>();
22.         matched = false;
23.     }
24.     void OnMouseDown()
25.     {
26.         Debug.Log("Static CardBack : " + CardGame.CardBack);
27.         Debug.Log("cardback : " + cardIdName);
28.         CardGame.count++;
29.  
30.         Debug.Log("count = " + CardGame.count);
31.         if (CardGame.count == 1)
32.         {
33.             //not the same
34.             Debug.Log("not 2 picked");
35.             CardGame.CardBack = cardIdName;
36.         }
37.         else
38.         {
39.             // Check if they are the same
40.             Debug.Log("check if same");
41.             tempGame.CheckWin(cardIdName); //null reference xception
42.         }
43.  
44.  
45.  
46.     }
47.  
48.     public bool GetMatched()
49.     {
50.         return matched;
51.     }
52.  
53.     public bool GetIsFaceUp()
54.     {
55.         return faceUp;
56.     }
57.  
58.     public void SeMatchedToTrue()
59.     {
60.         matched = true;
61.     }
62.  
63.     public void SetCardSprite(Sprite newSprite)
64.     {
65.         CardBack = newSprite;
66.     }
67.  
68.     public string getCardBack()
69.     {
70.         return cardIdName;
71.     }
72. }
73.  

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CardGame : MonoBehaviour
6. {
7.  
8.     public static string CardBack = "null";
9.     public static int count = 0;
10.     private CardControllerScript[] cards;
11.     public string cardback;
12.  
13.     // Use this for initialization
14.     void Start()
15.     {
16.         CardBack = "null";
17.         cards = this.GetComponentsInChildren<CardControllerScript>();
18.         Debug.Log(cards.Length);
19.     }
20.  
21.     // Update is called once per frame
22.     void Update()
23.     {
24.  
25.     }
26.  
27.     public void CheckWin(string secondCard)
28.     {
29.         Debug.Log("CheckWin : " + CardBack + " : " + secondCard);
30.         if (count == 2)
31.         {
32.             if (CardBack == secondCard)
33.             {
34.                 Debug.Log("Matched!");
35.                 count = 0;
36.                 for (int i = 0; i < cards.Length; i++)
37.                 {
38.  
39.                     if (cards[i].getCardBack() == secondCard)
40.                     {
41.                         Debug.Log("SeMatchedToTrue");
42.                         cards[i].SeMatchedToTrue();
43.                     }
44.  
45.                 }
46.                 Debug.Log("you got 1 point");
47.             }
48.             else
49.             {
50.                 Debug.Log("Not match!" + cards.Length);
51.                 count = 0;
52.                 for (int i = 0; i < cards.Length; i++)
53.                 {
54.  
55.                     if (cards[i].GetMatched() == false)
56.                     {
57.                         cards[i].gameObject.SetActive(true);
58.                     }
59.                     CardBack = "null";
60.                 }
61.             }
62.         } // if count = 2
63.     }
64.  
65. }
66.  
67.  

 

do you have discord or any thing to chat?

 

when i play it and i click on a card it shows this

 

skype
i can make a discord

 

line 35 error,
Did you set the string to something in unity on each card gameobject?

 

it is line 41 and Do i write it in the box next to the script
EDIT: just made a discord