c# - Load more items into LonListSelector when scrolled to bottom in windows phone 8 -

i new in windows phone development , try load more items lonlistselector when scrolled bottom in windows phone 8.

i use code scrollviewer throw null value exception.

scrollviewer scrollviewer; private void mylist_loaded(object sender, routedeventargs e) {     //get scrollviewer listbox      scrollviewer = getscrollviewer(this.mylonglistselector);     //attach custom binding 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 pull out scrollviewer public static scrollviewer getscrollviewer(dependencyobject depobj) {     if (depobj scrollviewer) return depobj scrollviewer;      (int = 0; < 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 check if values 0.0 because both set when scrollviewer loads     if ((scrollviewer.scrollableheight <= scrollviewer.verticaloffset)         && (scrollviewer.scrollableheight != 0.0 && scrollviewer.verticaloffset != 0.0))     {         //the scrollbar @ bottom, load more results.      } } 

please me.