The foreach identifier and closures

Question

In the two following snippets, is the first one safe or must you do the second one?

By safe I mean is each thread guaranteed to call the method on the Foo from the same loop iteration in which the thread was created?

Or must you copy the reference to a new variable "local" to each iteration of the loop?

var threads = new List<Thread>();
foreach (Foo f in ListOfFoo)
{      
    Thread thread = new Thread(() => f.DoSomething());
    threads.Add(thread);
    thread.Start();
}

-

var threads = new List<Thread>();
foreach (Foo f in ListOfFoo)
{      
    Foo f2 = f;
    Thread thread = new Thread(() => f2.DoSomething());
    threads.Add(thread);
    thread.Start();
}

Update: As pointed out in Jon Skeet's answer, this doesn't have anything specifically to do with threading.

Answer 1

Pop Catalin and Marc Gravell's answers are correct. All I want to add is a link to my article about closures (which talks about both Java and C#). Just thought it might add a bit of value.

EDIT: I think it's worth giving an example which doesn't have the unpredictability of threading. Here's a short but complete program showing both approaches. The "bad action" list prints out 10 ten times; the "good action" list counts from 0 to 9.

using System;
using System.Collections.Generic;

class Test
{
    static void Main() 
    {
        List<Action> badActions = new List<Action>();
        List<Action> goodActions = new List<Action>();
        for (int i=0; i < 10; i++)
        {
            int copy = i;
            badActions.Add(() => Console.WriteLine(i));
            goodActions.Add(() => Console.WriteLine(copy));
        }
        Console.WriteLine("Bad actions:");
        foreach (Action action in badActions)
        {
            action();
        }
        Console.WriteLine("Good actions:");
        foreach (Action action in goodActions)
        {
            action();
        }
    }
}