NavigationContext.QueryString.TryGetValue NullReferenceException in WP7.5

Question

I would like to pass a string between to pages in 7.5

I have read some guides, but, I have a NullReferenceException .

Page.xml.cs :

 var item = ListBoxTiers.SelectedItem as CTiers; 
 NavigationService.Navigate(new Uri("/DetailTiers.xaml?selectedItem=" + item.m_strCode, UriKind.Relative));

If i look at the debugger, i can see : "DetailTiers.xaml?selectedItem=C0000015"

In my page , Page2.xms.cs:

    public Page2()
    {
        InitializeComponent();
        string strCodeTiers =   string.Empty;
        if (NavigationContext.QueryString.TryGetValue("selectedItem",out strCodeTiers)) // Exception here
        {

        }

Anyone know where is my error ?

Answer 1

You shouldn't call this code from the constructor, as the NavigationContext isn't initialized yet. Use the OnNavigatedTo event instead:

protected override void OnNavigatedTo(NavigationEventArgs e)
{
    string strCodeTiers = string.Empty;

    if (NavigationContext.QueryString.TryGetValue("selectedItem",out strCodeTiers))
    {
         // Whatever
    }
}