C# Anonymous Function Scope in Unity3D

Question

I came across this puzzling behavior while making a simulation project in Unity3D.

Basically, I was creating anonymous functions in a loop using the variable being looped, and add all these functions to a queue for processing.
The curious case in Unity3D Monobehavior is that they are only called on the last looped variable.

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using System;
public class TestDist : MonoBehaviour {

    int counter = 0;

    // Use this for initialization
    void Start () {

        List<LOL> lols = new List<LOL>();
        lols.Add(new LOL(1));
        lols.Add(new LOL(2));
        lols.Add(new LOL(3));

        List<Func<int>> fs = new List<Func<int>>();
        foreach (LOL l in lols)
        {
            //****** Critical Section *********
            Func<int> func = () =>
            {
                Debug.Log(l.ID);   //Prints "3" 3 times instead of
                return 0;          // "1", "2", "3"
            };
            //****** Critical Section *********

            fs.Add(func);
        }
        foreach (Func<int> f in fs)
        {
            f();
        }
    }

    // Update is called once per frame
    void Update () {
    }
}

class LOL
{
    public long ID;
    public LOL(long id)
    {
        ID = id;
    }
}

The code works good in plain Visual Studio Console application, but fails on Unity3D. (Version 5.0) by printing the last value ("3") 3 times instead of 1,2,3.
I tried various methods to circumvent and the following worked for no apparent reason : Changing the Critical Section to the following block solves the issue:

LOL another = l;
Func<int> func = () =>
{
    Debug.Log(another.ID);
    return 0;
};

Will be delighted if someone can answer (Unity Bug Fix Team does not seem to care)

Community · Accepted Answer · 2017-05-23 10:27:31Z

0

You are facing the problem of closure.

See this question : What are 'closures' in C#?

Try this instead. Its merely the same, but not completely since you declare a "new" lol in your loop :

for( int index = 0 ; index < lols.Count ; ++index )
{
    Lol l = lols[index];
    Func<int> func = () =>
    {
        Debug.Log(l.ID);
        return 0;
    };
    fs.Add(func);
}

If it does not work, try :

foreach (LOL l in lols)
{           
    fs.Add(GetFunction(l.ID));
}

// ...

private Func<int> GetFunction( int identifier)
{     
    Func<int> func = () =>
    {
        Debug.Log(identifier);
        return 0;
    };
    return func ;
}

I does not feel comfortable with closure, I must admit.

edited May 23, 2017 at 10:27

CommunityBot

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answered Oct 20, 2016 at 14:48

Hellium

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