C# Is Locking Required When Swapping Variable Reference in Multithreaded Application

Question

I have an application where I want multiple threads to read a list. I want to update the list with new data periodically. When the list is updated, I figure I can create a new list and replace it with the old one. Example:

private List<string> _list = new List<string>();
private void UpdateList()
{
    var newList = new List<string>(QueryList(...));
    _list = newList;
}

private void ThreadRun()
{
    foreach (var item in _list)
    {
        // process item...
    }
}

In the UpdateList method, a new list is created and the _list reference is swapped with the new list. From my thinking, any existing thread will still hold the reference to the old list (which is OK for me), any new thread will pick up the new list. Eventually, all threads will end and the old list will be eventually garbage collected. Is there any locking required in this code, or is there anything I need to take care of to ensure safe multi-threaded access?

Answer 1

To be sure neither staleness nor optimisations hurt you, use Interlocked.Exchange() to update the field. Then you'll have an appropriate memory barrier on the write, without the expense of having volatile on every read.

Answer 2

You're making instances of the list immutable and it is only mutable state that causes problems.

Your code is fine, but you should consider marking _list as volatile. This would mean that all reads get the latest version and the compiler / jitter / cpu won't optimise away any reads.

Alternatively, you could put a Thread.MemoryBarrier() just before you assign the new list to make sure that all writes to the new list are committed before you publish it, but it's not a problem on x86 architecture.