List.Add() thread safety

Question

I understand that in general a List is not thread safe, however is there anything wrong with simply adding items into a list if the threads never perform any other operations on the list (such as traversing it)?

Example:

List<object> list = new List<object>(); Parallel.ForEach(transactions, tran => {     list.Add(new object()); }); 

Answer 1

Behind the scenes lots of things happen, including reallocating buffers and copying elements. That code will cause danger. Very simply, there are no atomic operations when adding to a list, at the least the "Length" property needs to be updates, and item needs to be put in at the right location, and (if there's a separate variable) the index needs to be updated. Multiple threads can trample over each other. And if a grow is required then there is lots more going on. If something is writing to a list nothing else should be reading or writing to it.

In .NET 4.0 we have concurrent collections, which are handily threadsafe and don't require locks.

Answer 2

You current approach is not thread-safe - I would suggest avoiding this altogether - since you basically do a data transformation PLINQ might be a better approach ( I know this is a simplified example but in the end you are projecting each transaction into another "state" object).

List<object> list = transactions.AsParallel()                                 .Select( tran => new object())                                 .ToList();