Is there a race condition in this common pattern used to prevent NullReferenceException?

Question

I asked this question and got this interesting (and a little disconcerting) answer.

Daniel states in his answer (unless I\'m readin

Answer 1

This code will not throw a null reference exception. This one is thread safe:

public void DoCallback() {
    Action local;
    local = Callback;
    if (local == null)
        local = new Action(() => { });
    local();
}

The reason this one is thread-safe, and can-not throw a NullReferenceException on Callback, is it's copying to a local variable before doing it's null check / call. Even if the original Callback was set to null after the null check, the local variable will still be valid.

However the following is a different story:

public void DoCallbackIfElse() {
    if (null != Callback) Callback();
    else new Action(() => { })();
}

In this one it's looking at a public variable, Callback can be changed to null AFTER the if (null != Callback) which would throw an exception on Callback();