Parallel.For List<Int32>

Question

I wrote the following lines of code:

    private static List<Int32> GetRandomList_Serial()
    {
        List<Int32> returnValue = new List<int>();
        Random random = new Random();

        for (int i = 0; i < 10000000; i++)
        {
            returnValue.Add(random.Next());
        }
        returnValue.Sort();
        return returnValue;
    }

I then wrote this block of code:

    private static List<Int32> GetRandomList_Parallel()
    {
        List<Int32> returnValue = new List<int>();
        Random random = new Random();

        Parallel.For(0, 10000000, y =>
        {
            returnValue.Add(random.Next());
        });

        returnValue.Sort();
        return returnValue;

    }

Serial works fine, Parallel throws this exception:

System.ArgumentException was unhandled by user code

Destination array was not long enough. Check destIndex and length, and the array's lower bounds.

Anyone have an idea why?

Answer 1

You're using a List<> which is not thread safe. Use ConcurrentBag<>. I run into this all the time while switching to Parallel loops. It will happen intermittently, not every time so it's hard to detect.