How to Check/uncheck all ietms in listbox in Windopws phone

Question

Hello
I have a listbox in windows phone and i want had bind it with all contacts in my Phone. I want to check/uncheck all items in listbox on button click. My XML code is 

 <ListBox Name="lstcontacts" ItemsSource="{Binding}" Margin="10,93,10,113"  SelectionMode="Multiple" Foreground="#FF333333" >

                <ListBox.ItemTemplate>

                    <DataTemplate>

                        <StackPanel Orientation="Horizontal">

                            <StackPanel Orientation="Horizontal" Tag="{Binding ObjectID}">

                                <CheckBox x:Name="contactChk" Tag="{Binding}" IsChecked="true" IsEnabled="True" Foreground="Black" Background="Black" BorderBrush="Black" Checked="chbox_checked" />

                                <TextBlock Name="txtResults1" Text="{Binding Path=DisplayName, Mode=TwoWay}"  />

                            </StackPanel>

                        </StackPanel>

                    </DataTemplate>

                </ListBox.ItemTemplate>

            </ListBox>

            

            

            <Button Content="Check All" HorizontalAlignment="Left" Margin="0,499,0,0" VerticalAlignment="Top" Click="Button_Click_1"/>

            <Button Content="Uncheck All" HorizontalAlignment="Left" Margin="138,499,0,0" VerticalAlignment="Top"/>

            <Button Content="Backup" HorizontalAlignment="Left" Margin="304,499,0,0" VerticalAlignment="Top" Width="142"/>

            

And C# code is 

using System;

using System.Collections.Generic;

using System.Linq;

using System.Net;

using System.Windows;

using System.Windows.Controls;

using System.Windows.Navigation;

using Microsoft.Phone.Controls;

using Microsoft.Phone.Shell;

using Microsoft.Phone.Data;

using Microsoft.Phone.UserData;

using Microsoft.Phone.Tasks;

using System.Data;

using System.Windows.Input;

using System.Windows.Media;

using System.Windows.Media.Animation;

using System.Windows.Shapes;

using Microsoft.Phone.Controls;

namespace MyPhoneBackup

{

    public partial class CreateContactBackups : PhoneApplicationPage

    {

        //AddressChooserTask addressChooserTask;

        //PhoneNumberChooserTask addressChooserTask;

        

        public CreateContactBackups()

        {

            InitializeComponent();

            

            Contacts cContacts = new Contacts();

            cContacts.SearchCompleted += new EventHandler<ContactsSearchEventArgs>(ContactsSearch);

            cContacts.SearchAsync(String.Empty, FilterKind.DisplayName, null);

            /* addressChooserTask = new PhoneNumberChooserTask();

            addressChooserTask.Completed += new EventHandler<PhoneNumberResult>(addressChooserTask_Completed); 

            //addressChooserTask.Completed += new EventHandler<AddressResult>(addressChooserTask_Completed); */

        }

        /*

         void addressChooserTask_Completed(object sender, PhoneNumberResult e)

         {

             if (e.TaskResult == TaskResult.OK)

             {

                 txtDisplay.Text = "The address for " + e.DisplayName + " is " + e.PhoneNumber;

             }

         }*/

        private void Button_Click_2(object sender, RoutedEventArgs e)

        {

            /*try

            {

                addressChooserTask.Show();

            }

            catch (System.InvalidOperationException ex)

            {

            }*/

        }

        void ContactsSearch(object sender, ContactsSearchEventArgs e)

        {

            try

            {

                lstcontacts.DataContext = e.Results;

            }

            catch (System.Exception)

            {

                txtResults.Text = "No Results Available";

            }

            if (lstcontacts.Items.Any())

            {

                txtResults.Text = "Below is the List of Contacts";

            }

            else

            {

                txtResults.Text = "No Results Available";

            }

        }

        private void Button_Click_1(object sender, RoutedEventArgs e)

        {

                       //CheckBox contactChk = lstcontacts.SelectedItem as CheckBox;

           // SearchVisualTree(this.lstcontacts);

           // int count22 = lstcontacts.Children.Count;

            //StackPanel FirstPanel = GetChild<StackPanel>(lstcontacts) as StackPanel;

            //StackPanel SecondPanel = GetChild<StackPanel>(FirstPanel) as StackPanel;

            //int i = lstcontacts.Items.Count;

            //for (int j = 0; j <= i; j++ )

            //{

            //    ListBoxItem item = this.lstcontacts.ItemContainerGenerator.ContainerFromIndex(Convert.ToInt32(j)) as ListBoxItem;

            //    CheckBox contactChk = FindFirstElementInVisualTree<CheckBox>(item);

            //    contactChk.IsChecked = false;

            //}

            StackPanel FirstPanel = GetChild<StackPanel>(lstcontacts) as StackPanel;

            StackPanel SecondPanel = GetChild<StackPanel>(FirstPanel) as StackPanel;

            for (int n = 0; n < SecondPanel.Children.Count; n++)

            {

               // CheckBox CheckBoxItem = GetChild<CheckBox>(SecondPanel);

                ListBoxItem item = this.lstcontacts.ItemContainerGenerator.ContainerFromIndex(Convert.ToInt32(n)) as ListBoxItem;

                CheckBox CheckBoxItem = FindFirstElementInVisualTree<CheckBox>(item);

                CheckBoxItem.IsChecked = false;

            }

        }

        /**/

        public T GetChild<T>(DependencyObject obj) where T : DependencyObject

        {

            DependencyObject child = null;

            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)

            {

                child = VisualTreeHelper.GetChild(obj, i);

                if (child != null && child.GetType() == typeof(T))

                    break;

                else if (child != null)

                {

                    child = GetChild<T>(child);

                    if (child != null && child.GetType() == typeof(T))

                        break;

                }

            }

            return child as T;

        }

        /**/

        private void SearchVisualTree(DependencyObject targetElement)

        {

            var count = VisualTreeHelper.GetChildrenCount(targetElement);

            if (count == 0)

                return;

            for (int i = 0; i < count; i++)

            {

                var child = VisualTreeHelper.GetChild(targetElement, i);

                if (child is TextBlock)

                {

                    TextBlock myItems = (TextBlock)child;

                    if (myItems.Text == "Item2")                    {

                        myItems.Foreground = new SolidColorBrush(Colors.Green);

                        return;

                    }

                }

                else

                {

                    SearchVisualTree(child);

               }

            }

        }

        private T FindFirstElementInVisualTree<T>(DependencyObject parentElement) where T : DependencyObject

        {

            var count = VisualTreeHelper.GetChildrenCount(parentElement);

            if (count == 0)

                return null;

            for (int i = 0; i <= count; i++)

            {

                var child = VisualTreeHelper.GetChild(parentElement, i);

                if (child != null && child is T)

                {

                    return (T)child;

                }

                else

                {

                    var result = FindFirstElementInVisualTree<T>(child);

                    if (result != null)

                        return result;

                }

            }

            return null;

        }

        public void chbox_checked(object sender, RoutedEventArgs e) {

            

        }

   

    }

   

}

In this code only first two items in list box are being checked/uncheck on button click. Please help me how can i check/uncheck all items in list on button click