C＃Parallel.ForEach抛出indexOutOfBoundException

Question

I'm using a lot of parallel foreach loops. they are mostly used for elaborating datas and after add them to a list.

this is an example of my methods:

System.Threading.Tasks.Parallel.ForEach(magazzini, m =>
{
    try
    {
        warehouse.Add(new WAREHOUSE()
        {
            ID = m.Id,
            NAME = m.new_name,
            CODE = m.new_codice,
            INTEGRATION_KEY = m.dynamics_integrationkey,
            RESOURCE_ID = m.new_mobileuser == null ? Guid.Empty : m.new_mobileuser.Id
        });
    }
    catch (Exception ex)
    {
        Logging.Error(authToken, string.Format("GetAllMagazzini|Magazzini|{0}", ex.InnerException));
    }
});

I read in this old post that it might be caused by the not thread-safe property of the List<T> . My problem is the following:

I have those not only for loops like this but also for long loops with a lot of operations. I had to use the parallel.foreach because using the classic foreach makes the query to timeout, since it's on a WebService.

How can I use the parallel.foreach avoiding the indexOutOfBoundException ? I have to change the List<T> in something else?

Answer 1

Yes, change List<T> to ConcurrentBag<T> since the resizing of the backing array in List<T> will cause problems with concurrent write operations. Another option is to create a lock around the add operation.

If there are no other operations that adding in your Parallel.ForEach concurrency is not necessary and will even negatively impact performance. If that is the case, create a regular foreach or use AddRange .