C# 在windows phone 8中滚动到底部时，将更多项目加载到LonListSelector中

我是windows phone开发新手，在windows phone 8中滚动到底部时，我尝试将更多项目加载到LonListSelector中

我使用这段代码，但scrollViewer抛出空值异常

ScrollViewer scrollViewer;
private void Mylist_Loaded(object sender, RoutedEventArgs e)
{
    //get the ScrollViewer from the ListBox
     scrollViewer = GetScrollViewer(this.MyLongListSelector);
    //attach to custom binding to check if ScrollViewer verticalOffset property has changed
    var binding = new Binding("VerticalOffset") { Source = scrollViewer };
    var offsetChangeListener = DependencyProperty.RegisterAttached(
        "ListenerOffset",
        typeof(object),
        typeof(UserControl),
        new PropertyMetadata(OnScrollChanged));
    scrollViewer.SetBinding(offsetChangeListener, binding);
}
// method to pull out a ScrollViewer
public static ScrollViewer GetScrollViewer(DependencyObject depObj)
{
    if (depObj is ScrollViewer) return depObj as ScrollViewer;

    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
    {
        var child = VisualTreeHelper.GetChild(depObj, i);

        var result = GetScrollViewer(child);
        if (result != null) return result;
    }
    return null;
}
public void OnScrollChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
{
    //We have to check if the values are 0.0 because they are both set to this when the scrollviewer loads
    if ((scrollViewer.ScrollableHeight <= scrollViewer.VerticalOffset)
        && (scrollViewer.ScrollableHeight != 0.0 && scrollViewer.VerticalOffset != 0.0))
    {
        //The ScrollBar is at the bottom, load more results.

    }
}

ScrollViewer；
已加载私有void Mylist_（对象发送方，路由目标）
{
//从列表框中获取ScrollViewer
scrollViewer=GetScrollViewer（this.MyLongListSelector）；
//附加到自定义绑定以检查ScrollViewer verticalOffset属性是否已更改
var binding=new binding（“VerticalOffset”）{Source=scrollViewer}；
var OffsetAngelistener=DependencyProperty.RegisterAttached(
“ListenerOffset”，
类型（对象），
类型（用户控制），
新属性元数据（OnScrollChanged））；
scrollViewer.SetBinding（offsetChangeListener、binding）；
}
//方法来拉出ScrollViewer
公共静态ScrollViewer GetScrollViewer（DependencyObject depObj）
{
如果（depObj是ScrollViewer），则返回depObj作为ScrollViewer；
for（int i=0；i