How to handle list picker in Wp7

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

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?

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

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();                   

}