List<T> multiple writer thread safety

Question

If I have multiple threads calling the Add method of a List object, and no readers, do I only need to lock on the List object before calling Add to be thread safe?

Answer 1

Usually it's best to lock on a separate (immutable) object... locking on the same object you're modifying is bad practice should be done with caution.

private readonly object sync = new object();
private List<object> list = new List<object>();

void MultiThreadedMethod(object val)
{
    lock(sync)
    {
        list.Add(val);
    }
}

In a basic case like this you will not have a problem, but if there is a possibility that your list can be changed (not the contents of the list, but the list itself), then you might have a situation where you lock on two objects when you only intend to lock on one.