设置FetchOnDemand=true就会在你需要的时候自动取下一个数据包，帮助中这样说：
By default FetchOnDemand is True, meaning that the client dataset fetches additional packets of data as needed (for example, as a user scrolls through data, or conducts a search).
如果要取记录总数的话可以通过参数从远程方法中返回。
