Usage of ?? operator (null-coalescing operator)

Usage of ?? operator (null-coalescing operator)

I use ?? operator very heavily in my code. But today I just came across a
question.
Here the code which I use for ?? operator
private List<string> _names;
public List<string> Names
{
get { return _names ?? (_names = new List<string>()); }
}
But in some locations I have seen this code as well.
private List<string> _names;
public List<string> Names
{
get { return _names ?? new List<string>(); }
}
What is real difference between these codes. In one I am assigning _names
= new List() while in other I am just doing new List().