Parallel code throws OutOfRangeException

Question

Does anyone know how why this code returns out of range exception?
For example if the leastAbstractions List instance has count == 10, the loop will execute 11 times finishing with i = 10 and returning this exception.

 for (int i = 0; i < leastAbstractions.Count; i++)
 {
     Task.Factory.StartNew((object state) =>
     {
         this.Authenticate(new HighFragment(leastAbstractions[i])).Reactivate();
     }, TaskCreationOptions.PreferFairness);
 }

Jon Skeet · Accepted Answer · 2011-01-29 12:07:44Z

4

Your loop isn't actually executing 11 times - it's only executing 10 times, but i == 10 by the time some of those tasks execute.

It's the normal problem - you're capturing a loop variable in a lambda expression. Just take a copy of the counter, and capture that instead:

for (int i = 0; i < leastAbstractions.Count; i++)
 {
     int copy = i;
     Task.Factory.StartNew((object state) =>
     {
         this.Authenticate(new HighFragment(leastAbstractions[copy]))
                                     .Reactivate();
     }, TaskCreationOptions.PreferFairness);
 }

That way, when your task executes, you'll see the current value of the "instance" of copy that you captured - and that value never changes, unlike the value of i.

See Eric Lippert's blog posts on this: part 1; part 2.

edited Jan 29, 2011 at 12:07

answered Jan 29, 2011 at 12:00

Jon Skeet

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2 Comments

pevogam Over a year ago

I thought that the counter is passed as an argument immediately in the creation of the new task. I guess I was wrong. Thanks.

Jon Skeet Over a year ago

@blaze: You're using the counter in the lambda expression. When the lambda expression is executed, the current value of whatever you're capturing is used. That has to happen in a different thread, because that lambda expression is what your task is meant to do.

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