List.Add() thread safety

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());
});

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.

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();