Threads and IndexOutOfRange exception

Question

Note that in the locals i = 3

As you can clearly see the highest index of the args[] is 2, however the iterator somehow gets to 3. Explanation?

Edit: The commented Thread.Sleep magically fixes the problem.

there are two threads if i am not mistaken...the main thread and temp...the main thread controls the for loop..and temp[] controls the string array...the `Thread.Sleep(1)` fixes the problem by forcing the main thread to sleep for 1 millisecond..i am i right? — HackerMan, Apr 09 '14 at 00:41
You should remove your extra thread and verify you get the exception. Thread testing 101! — Erik Philips, Apr 09 '14 at 00:45
This is a matter of the lambda capturing the index, `i`. It has little to do with threads. — John Saunders, Apr 09 '14 at 00:46

score 6 · Accepted Answer · answered Apr 09 '14 at 00:46

6

This is caused by i being declared outside of the for loop, for the entire for loop. Since there is no guarantee that the Thread is executed at that point, the value of i can change before the Thread executes. You can "resolve" this by declaring a local variable in the for loop.

//for loop ..
var localIndex = i;
var temp = new Thread(() => PrintOut(args[localIndex], IsFilePath(args[localIndex])));
temp.Start();
//for loop ..

EDIT: Also, can you please post a code snippet next time, saves me having to write out the code again :P

answered Apr 09 '14 at 00:46

Xenolightning

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Right, the `for` loop's incrementor (`i++`) executes before the body of the thread is invoked. Thus, `i` can be incremented to `3` before the thread actually runs. – Kirk Woll Apr 09 '14 at 00:47
Always thought the call to Thread.Start actually starts the thread. "Once a thread is in the ThreadState.Running state, the operating system can schedule it for execution." http://msdn.microsoft.com/en-us/library/6x4c42hc%28v=vs.110%29.aspx – TheDespite Apr 09 '14 at 02:16

score 1 · Answer 2 · edited Apr 24 '17 at 23:42

First of all

for (var i = 0; i< args.Length; i++)
{
}

is equivalent to:

int i = 0;

loop:

if (i < args.Length)
{
    i++;
    goto loop;
}

so you see that i is incremented to 3 in order to check your condition.

Of course, new Thread(() => PrintOut(args[i], IsFilePath(args[i]))) is never actually called when i is 3. However, since there is only one instance of i which the loop changes on each iteration, the execution that started when i was 2 is affected when i is incremented to 3.

To fix this, you need to copy the value of i into a new variable on each loop iteration. This copy will not be updated by the for loop:

for (var i = 0; i< args.Length; i++)
{
     var copy = i;
     Thread temp = new Thread(() => PrintOut(args[copy], IsFilePath(args[copy]))
}

Threads and IndexOutOfRange exception

2 Answers2