Passing int parameters to delegate function becomes reference passing

I bind three delegate functions with parameter 0,1,2 to three buttons. Since int is value type, so i think the output should be 0,1,2. But it's not. The output is 3,3,3. It means they are referenced to the same i, which becomes 3 at last.

Can somebody tells me why it happens?

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEngine.UI;
5.  
6. public class TestScript : MonoBehaviour {
7.  
8.     public Button[] buttons;
9.  
10.     // Use this for initialization
11.     void Start () {
12.         for(int i=0; i < 3; i++){
13.             int index = new int();
14.             index = i;
15.             buttons[i].onClick.AddListener(delegate { OnSelect(index); });
16.         }
17.     }
18.    
19.     // Update is called once per frame
20.     void Update () {
21.        
22.     }
23.  
24.     private void OnSelect(int index=0){
25.         Debug.Log(index);
26.     }
27. }

 

Anonymous delegates capture the entire context of their local function call. Try calling something like...

Code (csharp):

1.  
2. void AddListener(Button button, int parameter)
3. {
4.    button.onClick.AddListener(delegate { OnSelect(parameter);  });
5. }
6.  

... in your loop as a workaround. That should create a different local context.

Honestly, I would just move away from anonymous delegates entirely and create your logic in a different way. They're very tricky to control the lifetime and context of, and are hard to debug.

 

You are right. Thanks for your help.

Actually i do in this way because i have multiple buttons which should have similar onclick event, just with different parameters(like the one show before). Is there any better way to realize this functionalities?