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

Yes, you need to lock. Instance methods are not guaranteed to be thread safe on List<T>.

Answer 2

Yes. But you might also consider subclassing the List and "new" over the Add method. That will allow you to encapsulate the lock. It will work great as long as nothing accesses the base List. This technique is used for simple tree structures in XNA video games.

Answer 3

I think these have been linked before on here, but I found them to be very useful and interesting:

Thread safe collections are hard

Thread safe collection

Answer 4

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.