Parallel Foreach array from async method

Question

I am getting an error when looping array from async method as Parallel. When debuging, I can see count of the resultsArray is 11. But after stepping over a few times, it showed me "Source array was not long enough" Can I please get what the issue of my code?

public async Task<IActionResult> Generate(int id)
{
   List<Product> products = new List<Product>();
    Result[] resultArray = await Manager.GetResultArray();
    Parallel.ForEach(resultArray , result=> //Error here
    {
        SomeMethod(result)); // SomeMethod cast result to Produc class and add to products list
    });
    ...
}

Take a look at these two. stackoverflow.com/questions/42007593/… stackoverflow.com/questions/10362023/… — Efe Zaladin
– Efe Zaladin, Commented Jun 27, 2020 at 15:40
Can you please add code for your SomeMethod and Result class? — Guru Stron
– Guru Stron, Commented Jun 27, 2020 at 15:46
@GuruStron I can't put the exact code but added some changes...hope this helps — dbenbyeon
– dbenbyeon, Commented Jun 27, 2020 at 16:00

Guru Stron · Accepted Answer · 2020-06-27 16:07:15Z

1

List is not an thread safe collection, which you are trying to update from multiple threads. You can try to use ConcurrentBag in this case:

var products = new ConcurrentBag<Product>();
Result[] resultArray = await Manager.GetResultArray();
Parallel.ForEach(resultArray , result=> //Error here
{
    SomeMethod(result)); // SomeMethod should add to ConcurrentBag
});
...

answered Jun 27, 2020 at 16:07

Guru Stron

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1 Comment

dbenbyeon Over a year ago

Thanks Guru, I didn't know the List is not a thread safe. Could you please explain why the List is not a thread safe?

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