Getting Array Index Out of bound exception while adding elements to a List in Parallel.ForEach in C#

Question

I am trying to add elements in a List in C#. I am doing it in a Parallel.ForEach loop. I am getting Array index out of bound execption. What is the solution for this ?

var processes = new List<Process>();
Parallel.ForEach(productList, new ParallelOptions { MaxDegreeOfParallelism = 30 }, product =>
{
      // Some Logic               
      processes.Add(process);
}

A List<T> is not thread-safe. Use a ConcurrentBag<T> or lock around the call to Add (which makes it pretty meaningless to use a Parallel.ForEach in the first place). — mm8
– mm8, Commented Aug 29, 2019 at 11:23
Actually, introducing a lock shouldn't be that big of a problem. Sure, it'll make adding items to the list be a sequential operation, but I'm assuming the cost of doing that isn't the biggest thing you have in the parallel code, so most of it will still run in parallel. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Aug 29, 2019 at 11:33
True. its not the biggest thing i am doing in the loop. lock solution is also fine but i think delegating synchronization work to the framework itself is a better approach then adding lock explicitly in code :) — Nikhil Chopra
– Nikhil Chopra, Commented Aug 29, 2019 at 11:41

mm8 · Accepted Answer · 2019-08-29 11:27:06Z

3

A List<T> is not thread-safe. This means that you cannot call its Add method from more than one thread simultaneously and expect it to work.

You should replace the list with a ConcurrentBag<T>. The other option would be to synchronize the access to the list, for example using a lock statement. But if all you do is to simply add an item to the list in your loop, it doesn't make much sense to use a Parallel.ForEach and a List<T>.

answered Aug 29, 2019 at 11:27

mm8

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