使用 Parallel.Foreach 更新具有线程安全性的实体的属性

Question

I am trying to leverage Parallel.Foreach to update properties of entity: Contact as shown in the code sample

var contactIds = new List<int>();
contactIds.AddRange(res.RelatedContacts.Select(c => c.IdVal).ToList());
contacts = await _dbContext.Contacts.Where(co => contactIds.Contains(co.ContactId)).ProjectTo<ContactDTO>(_mapper.ConfigurationProvider).AsNoTracking().ToListAsync();

foreach (var contactrec in contacts)
{
    var contact = contacts.FirstOrDefault(c => c.ContactId.Equals(contactrec.ContactId));
    if (contact != null)
    {
        contact.address = addresses.FirstOrDefault(a => a.AddressId.Equals(contact.AddressId));
        contact.ImagePath = contact.ImageId > 0 ? images.FirstOrDefault(im => im.ImageId.Equals(contact.ImageId)).FilePath : "NA";
    }
}

I want to replace the above code with Parallel.Foreach with thread safety.

Answer 1

I agree with the comments, there are some code smells there and probably writing the query in a more efficient way would speed up the whole process more than any optimization obtained with parallelism.

Having said this, I think that you're looking for something like this:

var contactIds = new List<int>();
contactIds.AddRange(res.RelatedContacts.Select(c => c.IdVal).ToList());
contacts = await _dbContext.Contacts.Where(co => contactIds.Contains(co.ContactId)).ProjectTo<ContactDTO>(_mapper.ConfigurationProvider).AsNoTracking().ToListAsync();

Parallel.Foreach(contacts, contact => Update(contact));
//or: Parallel.Foreach(contacts, Update);

void Update(Contact contact){
    contact.address = addresses.FirstOrDefault(a => a.AddressId.Equals(contact.AddressId));
    contact.ImagePath = contact.ImageId > 0 ? images.FirstOrDefault(im => im.ImageId.Equals(contact.ImageId)).FilePath : "NA";
}

Some notes:

• no need to iterate through the Ids and then get the first element with that Id. Just iterate the elements.
• there shouldn't be any thread safe issue, the operations in Update are readonly.