List Index Out of Range exception when creating a task

Question

The exact error:

Index was out of range. Must be non-negative and less than the size of the collection.

I\'ve index arrays and l

Answer 1

You're the victim of accessing modified closure, as it's so succinctly called. Basically, since you're using a task - and a delegate to boot - the value of i is not guaranteed to be what it is you expect it to be. If you, however, copy i to a local variable, specific for the scope of one, single iteration, you should be fine.

for (int i = 0; i < addressList.Count; i++)
{
    textBox1.Text += ("Task for " + addressList[i] + ":" + portList[i] + " initiated." + Environment.NewLine);

    var iCopy = i;
    Task.Factory.StartNew(() => PingTaskAdapted(addressList[iCopy], portList[iCopy]));
}

However, as pointed out in this answer by nvoigt, it's far more clear when it comes to readability and maintainability if you copy the values which will be used rather than the iterator value.