帮我解释一下这个？？是什么意思，
protected SortedList AnsweredQuestions
{
get { return (SortedList)(ViewState[ "AnsweredQuestions "] ?? new SortedList()); }
set { ViewState[ "AnsweredQuestions "] = value; }
}

[解决办法]
??
这个符号在vs2005里头才有的，，是判断是否为可空类型！