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
 });
 ...
}

• Efe Zaladin: Take a look at these two. stackoverflow.com/questions/42007593/… stackoverflow.com/questions/10362023/…
• Guru Stron: Can you please add code for your SomeMethod and Result class?
• dbenbyeon: @GuruStron I can't put the exact code but added some changes...hope this helps

Answer 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
});
...

• dbenbyeon: 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?