Starting threads in a loop passing array elements to each

Question

i start 4 threads in a loop. each thread gets a reference to an array element to write the result.

But on the line where i create each thread, i get a System.IndexOutOfRangeException. I'm amazed that the index "i" is going out of range.

here is an example:

void ThreadsStarter()
{
    double[] data = new double[4];

    for(int i = 0; i < 4; i++)
    {
        Thread my_thread = new Thread(() => Work(data[i]));
        my_thread.Start();
    }
}

void Work(double data)
{
}

Why this is happening?

I've removed ref from your sample/title as unrelated to the question - feel free to edit/rollback. Note ref is generally hard to use/understand, especially when you start using more flexible collections like List<double> instead of Array. Try to avoid ref - i.e. in this case Tuple.Create(data, i) could be an option (obviously created before lambda to avoid problem you have now). — Alexei Levenkov
– Alexei Levenkov, Commented Jul 21, 2013 at 0:50
thanks. now i also know the name of that construction (closures/lambdas). i do try to avoid ref. i will consider using tuples instead, this seems cleaner. that's a good article Blorgbeard. — symbiont
– symbiont, Commented Jul 21, 2013 at 1:29

Sergey Kalinichenko · Accepted Answer · 2013-07-21 00:37:15Z

6

This is a common error: i gets evaluated when threads starts, which happens after the loop has ended. Make a temp, assign i to it, and use temp instead of i in your lambda to fix the issue:

void ThreadsStarter()
{
    double[] data = new double[4];

    for(int i = 0; i < 4; i++)
    {
        var temp = i;
        Thread my_thread = new Thread(() => Work(ref data[temp]));
        my_thread.Start();
    }
}

void Work(ref double data)
{
}

answered Jul 21, 2013 at 0:37

Sergey Kalinichenko

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1 Comment

BornToCode Over a year ago

This works because when you recreate a temp variable, the compiler detects that the closure is "attached" to a different variable on every loop pass and will generate a "new DisplayClass()" code (which encapsulates the closure) inside every pass. Without temp variable it'll use the same DisplayClass (which holds a member of the i variable) from within all loop passes. (Similar to the code that c# 5 generates for a foreach loop - see an example here).

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