执行Command时，ViewModel List属性为null

Question

代码更容易解​​释：

    private List<MatchHistory> matchHistories;
    public List<MatchHistory> MatchHistories
    {
        get
        {
            return matchHistories;
        }
        set
        {
            matchHistories = value;
            OnPropertyChanged();
        }
    }

    public Command GetMatchHistoriesCmd
    {
        get;
    }

    private void ExecuteGetMatchHistories()
    {
        MatchHistories = Helper.GetMatchHistories();            // This returns List<MatchHistory>
    }

当它尝试分配给MatchHistories时调试它会中断，但是为什么GetMatchHistories返回一个对象时它会变为空？不应该只是将该对象分配给MatchHistories属性吗？

    public static List<MatchHistory> GetMatchHistories()
    {
        List<MatchHistory> mh = new List<MatchHistory>();
        mh.Add(new MatchHistory { match_id = "1234", right_win = true });
        return mh;
    }