Atomicity of C# Coalescing Operator

Question

I ran into some singleton code today in our codebase and I wasn't sure if the following was thread-safe:

public static IContentStructure Sentence{ 
    get {
       return _sentence ?? (_sentence = new Sentence()); 
    }
}

This statement is equivalent to:

if (_sentence != null) {
       return _sentence;
}
else {
    return (_sentence = new Sentence());
}

I believe that ?? is just a compiler trick and that the resulting code is still NOT atomic. In other words, two or more threads could find _sentence to be null before setting _sentence to a new Sentence and returning it.

To guarantee atomicity, we'd have to lock that bit of code:

public static IContentStructure Sentence{ 
    get {

       lock (_sentence) { return _sentence ?? (_sentence = new Sentence()); }
    }
}

Is that all correct?