Question ArgumentOutOfRangeException: Index was out of range.

Code (CSharp):

1. using System.Collections.Generic;
2. using UnityEngine;
3. using UnityEngine.UI;
4.  
5. public class ButtonManager : MonoBehaviour
6. {
7.     #region Variables
8.     [Header("Scroll Settings")]
9.     [SerializeField] private Scrollbar scrollbarHorizontal;
10.  
11.     [Header("Buttons Reference")]
12.     [SerializeField] private GameObject bottomPanel;
13.  
14.     private List<Button> bottomPanelButtonsList = new();
15.     private List<float> scrollbarValuesList = new();
16.  
17.     private float listCount;
18.  
19.     private const float scrollbarValueForShop = 0f;
20.     private const float scrollbarValueForLeaderboard = 0.25f;
21.     private const float scrollbarValueForHome = 0.5f;
22.     private const float scrollbarValueForFriends = 0.75f;
23.     private const float scrollbarValueForSettings = 1;
24.     #endregion
25.  
26.     #region In-Built Functions
27.     private void Awake()
28.     {
29.         scrollbarHorizontal.value = scrollbarValueForHome;  //To always start from Home
30.  
31.         PopulateButtonList();
32.         PopulateScrollbarValueList();
33.         //AddListenerToButtons();
34.     }
35.  
36.     private void Start()
37.     {
38.         //AddListenerToButtons();
39.         OnButtonClick();
40.     }
41.     #endregion
42.  
43.     #region Custom Functions
44.     private void PopulateButtonList()
45.     {
46.         if (bottomPanelButtonsList != null)
47.         {
48.             bottomPanelButtonsList.AddRange(bottomPanel.GetComponentsInChildren<Button>());
49.             Debug.Log("Button List Populated");
50.         }
51.         else
52.         {
53.             bottomPanelButtonsList = new();
54.         }
55.     }
56.  
57.     private void PopulateScrollbarValueList()
58.     {
59.         if (scrollbarValuesList != null)
60.         {
61.             scrollbarValuesList.Add(scrollbarValueForShop);
62.             scrollbarValuesList.Add(scrollbarValueForLeaderboard);
63.             scrollbarValuesList.Add(scrollbarValueForHome);
64.             scrollbarValuesList.Add(scrollbarValueForFriends);
65.             scrollbarValuesList.Add(scrollbarValueForSettings);
66.             Debug.Log("Float List Populated");
67.         }
68.         else
69.         {
70.             scrollbarValuesList = new();
71.         }
72.     }
73.  
74.     private void OnButtonClick()
75.     {
76.         if (bottomPanelButtonsList.Count == scrollbarValuesList.Count)
77.         {
78.             listCount = bottomPanelButtonsList.Count;
79.             Debug.Log($"{listCount}");  //5
80.             for (int i = 0; i < listCount; i++)
81.             {
82.                 //bottomPanelButtonsList[0].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[0]);
83.                 //bottomPanelButtonsList[1].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[1]);
84.                 //bottomPanelButtonsList[2].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[2]);
85.                 //bottomPanelButtonsList[3].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[3]);
86.                 //bottomPanelButtonsList[4].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[4]);
87.  
88.                 bottomPanelButtonsList[i].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[i]);
89.             }
90.         }
91.     }
92.     #endregion
93. }
94.  

Error Message:
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
System.Collections.Generic.List`1[T].get_Item (System.Int32 index) (at <9aad1b3a47484d63ba2b3985692d80e9>:0)
ButtonManager+<>c__DisplayClass14_0.<OnButtonClick>b__0 () (at Assets/Scripts/ButtonManager.cs:88)
UnityEngine.Events.InvokableCall.Invoke () (at <86acb61e0d2b4b36bc20af11093be9a5>:0)
UnityEngine.Events.UnityEvent.Invoke () (at <86acb61e0d2b4b36bc20af11093be9a5>:0)
UnityEngine.UI.Button.Press () (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/UI/Core/Button.cs:70)
UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/UI/Core/Button.cs:114)
UnityEngine.EventSystems.ExecuteEvents.Execute (UnityEngine.EventSystems.IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/ExecuteEvents.cs:57)
UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/ExecuteEvents.cs:272)
UnityEngine.EventSystems.EventSystem:Update() (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/EventSystem.cs:501)

 

A lambda expression captures a variable, not the value it holds when the lambda is created. In this case, after the loop runs, the value of i is 5. Therefore, every onClick listener you add uses the index 5 to access scrollbarValuesList when invoked, which obviously throws an error since it's out of range. To fix this, you need to create a local copy of i for every iteration and use it instead of i in the lambda.

Code (CSharp):

1.     private void OnButtonClick()
2.     {
3.         if (bottomPanelButtonsList.Count == scrollbarValuesList.Count)
4.         {
5.             listCount = bottomPanelButtonsList.Count;
6.             for (int i = 0; i < listCount; i++)
7.             {
8.                 int local = i;
9.                 bottomPanelButtonsList[i].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[local]);
10.             }
11.         }
12.     }

 

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