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

Edit: this all changes in C# 5, with a change to where the variable is defined (in the eyes of the compiler). From C# 5 onwards, they are the same.

---

Before C#5

The second is safe; the first isn't.

With foreach, the variable is declared outside the loop - i.e.

Foo f;
while(iterator.MoveNext())
{
     f = iterator.Current;
    // do something with f
}

This means that there is only 1 f in terms of the closure scope, and the threads might very likely get confused - calling the method multiple times on some instances and not at all on others. You can fix this with a second variable declaration inside the loop:

foreach(Foo f in ...) {
    Foo tmp = f;
    // do something with tmp
}

This then has a separate tmp in each closure scope, so there is no risk of this issue.

Here's a simple proof of the problem:

    static void Main()
    {
        int[] data = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        foreach (int i in data)
        {
            new Thread(() => Console.WriteLine(i)).Start();
        }
        Console.ReadLine();
    }

Outputs (at random):

1
3
4
4
5
7
7
8
9
9

Add a temp variable and it works:

        foreach (int i in data)
        {
            int j = i;
            new Thread(() => Console.WriteLine(j)).Start();
        }

(each number once, but of course the order isn't guaranteed)

Answer 2

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();
        }
    }
}

Answer 3

Your need to use option 2, creating a closure around a changing variable will use the value of the variable when the variable is used and not at closure creation time.

The implementation of anonymous methods in C# and its consequences (part 1)

The implementation of anonymous methods in C# and its consequences (part 2)

The implementation of anonymous methods in C# and its consequences (part 3)

Edit: to make it clear, in C# closures are "lexical closures" meaning they don't capture a variable's value but the variable itself. That means that when creating a closure to a changing variable the closure is actually a reference to the variable not a copy of it's value.

Edit2: added links to all blog posts if anyone is interested in reading about compiler internals.

Answer 4

This is an interesting question and it seems like we have seen people answer in all various ways. I was under the impression that the second way would be the only safe way. I whipped a real quick proof:

class Foo
{
    private int _id;
    public Foo(int id)
    {
        _id = id;
    }
    public void DoSomething()
    {
        Console.WriteLine(string.Format("Thread: {0} Id: {1}", Thread.CurrentThread.ManagedThreadId, this._id));
    }
}
class Program
{
    static void Main(string[] args)
    {
        var ListOfFoo = new List<Foo>();
        ListOfFoo.Add(new Foo(1));
        ListOfFoo.Add(new Foo(2));
        ListOfFoo.Add(new Foo(3));
        ListOfFoo.Add(new Foo(4));


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

if you run this you will see option 1 is definetly not safe.

Answer 5

In your case, you can avoid the problem without using the copying trick by mapping your ListOfFoo to a sequence of threads:

var threads = ListOfFoo.Select(foo => new Thread(() => foo.DoSomething()));
foreach (var t in threads)
{
    t.Start();
}

Answer 6

Both are safe as of C# version 5 (.NET framework 4.5). See this question for details: Has foreach's use of variables been changed in C# 5?

Answer 7

Foo f2 = f;

points to the same reference as

f 

So nothing lost and nothing gained ...