LongListSelectorExtended

using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Windows;
using System.Windows.Controls;
using Microsoft.Phone.Controls;

namespace LongListSelectorAdd
{
    public class ExtendedSelector : LongListSelector
    {
        private int _count;

        public static readonly DependencyProperty SelectedItemProperty =
            DependencyProperty.Register("SelectedItem", typeof(object), typeof(ExtendedSelector), new PropertyMetadata(default(object)));

        public static readonly DependencyProperty SelectionModeProperty =
            DependencyProperty.Register("SelectionMode", typeof(SelectionMode), typeof(ExtendedSelector), new PropertyMetadata(default(SelectionMode)));

        public SelectionMode SelectionMode
        {
            get { return (SelectionMode)GetValue(SelectionModeProperty); }
            set { SetValue(SelectionModeProperty, value); }
        }

        public new object SelectedItem
        {
            get { return GetValue(SelectedItemProperty); }
            set { SetValue(SelectedItemProperty, value); }
        }

        public ExtendedSelector()
        {
            SelectionMode = SelectionMode.Single;

            SelectionChanged += (sender, args) =>
            {
                if (SelectionMode == SelectionMode.Single)
                    SelectedItem = args.AddedItems[0];
                else if (SelectionMode == SelectionMode.Multiple)
                {
                    if (SelectedItem == null)
                    {
                        SelectedItem = new List<object>();
                    }

                    foreach (var item in args.AddedItems)
                    {
                        ((List<object>)SelectedItem).Add(item);
                    }

                    foreach (var removedItem in args.RemovedItems)
                    {
                        if (((List<object>)SelectedItem).Contains(removedItem))
                        {
                            ((List<object>)SelectedItem).Remove(removedItem);
                        }
                    }
                }
            };

            Loaded += (sender, args) =>
            {
                ((INotifyCollectionChanged)ItemsSource).CollectionChanged += (sender2, args2) =>
                {
                    if(ItemsSource.Count > 0)
                        ScrollTo(ItemsSource[0]);
                };
            };
        }
    }
}