The foreach identifier and closures

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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)

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