ArgumentOutOfRangeException: Index was out of range.

Unity Engine
1 
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class ButtonManager : MonoBehaviour
{
    #region Variables
    [Header("Scroll Settings")]
    [SerializeField] private Scrollbar scrollbarHorizontal;

    [Header("Buttons Reference")]
    [SerializeField] private GameObject bottomPanel;

    private List<Button> bottomPanelButtonsList = new();
    private List<float> scrollbarValuesList = new();

    private float listCount;

    private const float scrollbarValueForShop = 0f;
    private const float scrollbarValueForLeaderboard = 0.25f;
    private const float scrollbarValueForHome = 0.5f;
    private const float scrollbarValueForFriends = 0.75f;
    private const float scrollbarValueForSettings = 1;
    #endregion

    #region In-Built Functions
    private void Awake()
    {
        scrollbarHorizontal.value = scrollbarValueForHome;  //To always start from Home

        PopulateButtonList();
        PopulateScrollbarValueList();
        //AddListenerToButtons();
    }

    private void Start()
    {
        //AddListenerToButtons();
        OnButtonClick();
    }
    #endregion

    #region Custom Functions
    private void PopulateButtonList()
    {
        if (bottomPanelButtonsList != null)
        {
            bottomPanelButtonsList.AddRange(bottomPanel.GetComponentsInChildren<Button>());
            Debug.Log("Button List Populated");
        }
        else
        {
            bottomPanelButtonsList = new();
        }
    }

    private void PopulateScrollbarValueList()
    {
        if (scrollbarValuesList != null)
        {
            scrollbarValuesList.Add(scrollbarValueForShop);
            scrollbarValuesList.Add(scrollbarValueForLeaderboard);
            scrollbarValuesList.Add(scrollbarValueForHome);
            scrollbarValuesList.Add(scrollbarValueForFriends);
            scrollbarValuesList.Add(scrollbarValueForSettings);
            Debug.Log("Float List Populated");
        }
        else
        {
            scrollbarValuesList = new();
        }
    }

    private void OnButtonClick()
    {
        if (bottomPanelButtonsList.Count == scrollbarValuesList.Count)
        {
            listCount = bottomPanelButtonsList.Count;
            Debug.Log($"{listCount}");  //5
            for (int i = 0; i < listCount; i++)
            {
                //bottomPanelButtonsList[0].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[0]);
                //bottomPanelButtonsList[1].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[1]);
                //bottomPanelButtonsList[2].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[2]);
                //bottomPanelButtonsList[3].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[3]);
                //bottomPanelButtonsList[4].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[4]);

                bottomPanelButtonsList[i].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[i]);
            }
        }
    }
    #endregion
}

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.List1[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+EventFunction1[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)

2

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.

    private void OnButtonClick()
    {
        if (bottomPanelButtonsList.Count == scrollbarValuesList.Count)
        {
            listCount = bottomPanelButtonsList.Count;
            for (int i = 0; i < listCount; i++)
            {
                int local = i;
                bottomPanelButtonsList[i].onClick.AddListener(() => scrollbarHorizontal.value = scrollbarValuesList[local]);
            }
        }
    }
3

How do you use DoTween for ScrollReact?