How to handle list picker in Wp7 - windows-phone-7

Question

I have a list picker which is displayed in my phone application page.I have created list picker in starting of class,and i am adding the list picker in the phoneApplicationPage_loaded() method.When the page is launched the first time, ,the scenario works perfectly and its navigates further to second page.When i navigate back to previous page(containing list picker),it shows Invalid Operation Exception occured stating "Element is already the child of another element."
I want to know how to handle these scenarios?
Code is below
namespace My.Design
{
public partial class myclass : PhoneApplicationPage
{
String[] values = null;
ListPicker picker = new ListPicker();
StackPanel sp;
StackPanel mainFrame;
String statementInfo = "";
public myclass()
{
InitializeComponent();
}
private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
{
Debug.WriteLine("Phone Application Page Loaded_>>>>>>");
List<String> source = new List<String>();
displayUI();
}
public void displayUI()
{
Debug.WriteLine("About to display UI in miniStatement");
Debug.WriteLine("<-------------Data--------->");
Debug.WriteLine(statementInfo);
Debug.WriteLine("<-------------Data--------->");
int count = VisualTreeHelper.GetChildrenCount(this);
if (count > 0)
{
for (int i = 0; i < count; i++)
{
UIElement child = (UIElement)VisualTreeHelper.GetChild(this, i);
string childTypeName = child.GetType().ToString();
Debug.WriteLine("Elements in this Child" + childTypeName);
}
}
List<String> source = new List<String>();
String[] allParams = ItemString.Split('#');
source.Add("PleaseSelect");
for (int i = 0; i < allParams.Length; i++)
{
Debug.WriteLine("All Params Length" + allParams[i]);
if (!(allParams[i].Equals("") && (!allParams[i].Equals(null))))
{
if (values != null)
{
Debug.WriteLine("Values length" + values.Length);
values[values.Length] = allParams[i];
}
else
{
Debug.WriteLine("Allparams Length" + allParams[i]);
source.Add(allParams[i]);
}
}
}
//picker = new ListPicker();
this.picker.ItemsSource = source;
mainFrame = new StackPanel();
TextBlock box = new TextBlock();
box.Text = "> DEmoClass";
box.FontSize = 40;
mainFrame.Children.Add(box);
Canvas canvas = new Canvas();
StackPanel sp = new StackPanel();
TextBlock box1 = new TextBlock();
box1.Text = "Number";
box1.HorizontalAlignment = HorizontalAlignment.Center;
box1.FontSize = 40;
SolidColorBrush scb1 = new SolidColorBrush(Colors.Black);
box1.Foreground = scb1;
sp.Children.Add(box1);
picker.Width = 400;
picker.Height = 150;
sp.Children.Add(picker);
Canvas.SetTop(sp, 150);
canvas.Children.Add(sp);
mainFrame.Children.Add(canvas);
this.ContentPanel1.Children.Add(mainFrame);
}
protected override void OnNavigatingFrom(System.Windows.Navigation.NavigatingCancelEventArgs e)
{
/*
Debug.WriteLine("OnNavigatingFrom>>>.>>MainPage");
if (sp != null)
{
sp.Children.Remove(picker);
}*/
base.OnNavigatingFrom(e);
}
}
}

Answer 1

If you are not intending to update the listpicker after navigating back from the second page add the following line in your Loaded event handler
private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
{
this.Loaded -= PhoneApplicationPage_Loaded;
Debug.WriteLine("Phone Application Page Loaded_>>>>>>");
List<String> source = new List<String>();
displayUI();
}

Answer 2

i don't know why you can not use that case when app resume from tombstoned.
error happened because when you back to your page , loaded event runs again.
by the way,
Application_Activated 's argument can tell you app resumes from tombstoned or not--.
if (e.IsApplicationInstancePreserved)
{
IsTombstoning = false;
}
else
{
IsTombstoning = true;
}

Answer 3

I'm curious why you're creating it in code and not leaving it in XAML? Also the error is coming from the fact that you're attempting to add it twice into a location that can probably only have a single content element. What's the higher level problem you're trying to solve?