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 updated, 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.