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 operation

Answer 1

It's not an unreasonable thing to ask. There are cases where methods which can cause thread-safety issues in combination with other methods are safe if they are the only method called.

However, this clearly isn't a case of it, when you consider the code shown in reflector:

public void Add(T item)
{
    if (this._size == this._items.Length)
    {
        this.EnsureCapacity(this._size + 1);
    }
    this._items[this._size++] = item;
    this._version++;
}

Even if EnsureCapacity was in itself threadsafe (and it most certainly is not), the above code is clearly not going to be threadsafe, considering the possibility of simultaneous calls to the increment operator causing mis-writes.

Either lock, use ConcurrentList, or perhaps use a lock-free queue as the place multiple threads write to, and the read from it - either directly or by filling a list with it - after they have done their work (I'm assuming that multiple simultaneous writes followed by single-threaded reading is your pattern here, judging from your question, as otherwise I can't see how the condition where Add is the only method called could be of any use).