Lambda expression cannot hold parameter

October 5, 2022

[SerializeField] List<Button> Buttons;
void Start()
{
    for (int i = 0; i < Buttons.Count; i++)
    {
        Buttons[i].onClick.AddListener(() => DoThing(i));// can't hold i variable
        //when click the each button result is always 3
        // i want 0,1,2
        print(i); // 0,1,2
    }
}
public void DoThing(int value)
{
    print(value);
}

I want to use i as parameter of a button click event action but each button always prints 3. Instead of using i Buttons[i].onClick.AddListener(() => DoThing(2)) is working fine. how use i as parameter

>Solution :

In your code, DoThing will capture the variable i (and not its value) and will only get the value of i when you invoke onClick.

Copy the value of a changing variable to the scope where the closure is defined

[SerializeField] List<Button> Buttons;

void Start()
{
    for (int i = 0; i < Buttons.Count; i++)
    {
        var iLocal = i;
        Buttons[i].onClick.AddListener(() => DoThing(iLocal));
    }
}

public void DoThing(int value)
{
    print(value);
}

This fix makes sure that when you get the context where the action was created, iLocal will hold the value corresponding to the index of the button in the list.