Static List assignment in C# is atomic

According to this reference assignment is atomic so why is Interlocked.Exchange(ref Object, Object) needed? Reference assignment is guaranteed to be atomic on all .NET platforms. Will this code is atomic,

public static List<MyType> _items;

public static List<MyType> Items
{
    get
    {
        if (_items== null)
        {
            _items= JsonConvert.DeserializeObject<List<MyType>>(ConfigurationManager.AppSettings["Items"]);
        }
        return _items;
    }
}

I know there might be multiple object as given here. But will it Items will be atomic(I mean it will be either null or List and not in middle)?