Async method with Parallel.ForEach

Question

I dont have much experiance with async methods and I am not sure if I am using parallel for each correctly. In some cases I am getting Index out of range exception on the line with: "listPages.Add"

private static int SetPages(FileDocument document, List<PageDTO> pages, string tableName, Entities entity, int accessID)
{
   List<Page> listPages = new List<Page>();
            
    Parallel.ForEach(pages, new ParallelOptions { MaxDegreeOfParallelism = 8 }, page =>
    {
        string docPath = GetPathString(page.PageID);
        if (File.Exists(docPath))
        {
            byte [] docBytes = ConvertToWebP(docPath);
            if(docBytes.Length > 0)
            {
                var fileStreamID = InsertInDatabase(docBytes, page.PageID, tableName);
                listPages.Add(SetPage(document, page.PageNumber, fileStreamID, tableName, accessID));
            }
        }
    });
            
    entity.FilePages.AddRange(listPages);
    return listPages.Count();
}

Your code isn't thread-safe. You should not be using shared mutable collections inside a `Parallel.ForEach` body. — Dai, Feb 14 '22 at 14:14
_"Async method with Parallel.ForEach"_ - No. Just no. Don't. ;D — Fildor, Feb 14 '22 at 14:16
`Parallel` were designed for CPU bound operations so do not use for I/O bound async operations. — Peter Csala, Feb 14 '22 at 14:18

score 1 · Accepted Answer · answered Feb 14 '22 at 14:16

Because your code might meet racing-condition there are a lot of thread action with List<Page> listPages = new List<Page>() , the List<> isn't thread safe collection

I would use ConcurrentBag<> instead of List<>, because ConcurrentBag<> is a thread safe collection.

Represents a thread-safe, unordered collection of objects.

private static int SetPages(FileDocument document, List<PageDTO> pages, string tableName, Entities entity, int accessID)
{
    ConcurrentBag<Page> listPages = new ConcurrentBag<Page>()
    Parallel.ForEach(pages, new ParallelOptions { MaxDegreeOfParallelism = 8 }, page =>
    {
        string docPath = GetPathString(page.PageID);
        if (File.Exists(docPath))
        {
            byte [] docBytes = ConvertToWebP(docPath);
            if(docBytes.Length > 0)
            {
                var fileStreamID = InsertInDatabase(docBytes, page.PageID, tableName);
                listPages.Add(SetPage(document, page.PageNumber, fileStreamID, tableName, accessID));
            }
        }
    });
            
    entity.FilePages.AddRange(listPages.ToList());
    return listPages.Count();
}

Async method with Parallel.ForEach

1 Answers1