public class PriorityListOfNodes
{
 
    private List<Node>          list_;
    private IComparer<Node>     sorter_;
 
    public List<Node> List_ {
            get {return list_;}
            set {list_ = value;}
    }
 
    public PriorityListOfNodes ()
    {
            sorter_ = new NodeSorter_fValueDescending ();
    }
 
    public Node PopFromEnd ()
    {
            Node temp = new Node (list_ [list_.Count - 1]);
            list_.RemoveAt (list_.Count - 1);
            return temp;
    }
 
    public Node PeekFromEnd ()
    {
            return list_ [list_.Count - 1];
    }
 
    public void Add (ref Node toAdd)
    {
            Debug.Log (toAdd);
            list_.Add (toAdd);
            list_.Sort (sorter_);
    }
}

我什么时候做

Node temp = new Node(10,20); //custom constructor
PriorityListOfNodes l = new PriorityListOfNodes();
l.add(temp);

我得到运行时异常：

Object reference not set to an instance of an object

我也试过没有参考,但结果相同.我在这做错了什么？