Are these two lines the same, '? … :' vs '??'?

Question

Is there a difference between these two lines?

MyName = (s.MyName == null) ? string.Empty : s.MyName

or

MyName = s.MyName ?? st         


        

Answer 1

If the property is more than a simple getter, you might be executing a function twice in the non-null case for the first one.

If the property is in a stateful object, then the second call to the property might return a different result:

class MyClass
{
    private IEnumerator<string> _next = Next();

    public MyClass()
    {
        this._next.MoveNext();
    }

    public string MyName
    {
        get
        {
            var n = this._next.Current;
            this._next.MoveNext();
            return n;
        }
    }


    public static IEnumerator<string> Next()
    {
        yield return "foo";
        yield return "bar";
    }
}

Also, in the non-string case, the class might overload == to do something different than the ternary operator. I don't believe that the ternary operator can be overloaded.