Question

I am building a WP7 app that consist on one listbox in the first page to show the id of several task and a detail page to show the details for each task (selected).

I need to pass the task id from the first page to the second one. I know that it is carried out by SelectionChanged event of the listbox.

This is my XAML code:

<phone:PhoneApplicationPage 
    x:Class="TaskListAlpha03.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
    xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" d:DesignWidth="480" d:DesignHeight="696"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    shell:SystemTray.IsVisible="True"    
    xmlns:controls="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone.Controls">

    <phone:PhoneApplicationPage.Resources>
        <DataTemplate x:Key="TaskListListBoxTemplate">            
            <StackPanel Orientation="Vertical" Margin="0,0,0,20">                
                <TextBlock Text="{Binding Crm_object_id}" FontSize="32" FontFamily="Segoe WP Bold" Foreground="Gray"/>
                <!--<TextBlock Text="{Binding Comment}" Margin="10,0,0,0"/>              -->
            </StackPanel>            
        </DataTemplate>
    </phone:PhoneApplicationPage.Resources>

        <!--LayoutRoot is the root grid where all page content is placed-->
    <Grid x:Name="LayoutRoot" Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <!--TitlePanel contains the name of the application and page title-->
        <StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,40">
            <TextBlock x:Name="ApplicationTitle" Text="TASK LIST ALPHA" Style="{StaticResource PhoneTextNormalStyle}"/>
            <TextBlock x:Name="PageTitle" Text="tasks" Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
        </StackPanel>

        <!--ContentPanel - place additional content here-->
        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
            <ListBox 
                        x:Name="allTaskListTasksListBox" 
                        ItemsSource="{Binding AllTaskListTasks}" 
                        ItemTemplate="{StaticResource TaskListListBoxTemplate}"
                        SelectionChanged="allTaskListTasksListBox_SelectionChanged" />
        </Grid>
    </Grid>


</phone:PhoneApplicationPage>

And this is the cs code:

    private void allTaskListTasksListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {            
        ListBoxItem lbi = ((sender as ListBox).SelectedItem as ListBoxItem);
        NavigationService.Navigate(new Uri("/View/Details.xaml?msg=" + lbi.Content.ToString(), UriKind.RelativeOrAbsolute));
    }

In the second page I have this sample code to check if it works:

protected override void OnNavigatedTo
        (System.Windows.Navigation.NavigationEventArgs e)
        {
            base.OnNavigatedTo(e);
            string msg = "";
            if (NavigationContext.QueryString.TryGetValue("msg", out msg))
                PageTitle.Text = msg;
        }

When I execute the application I have a “NullReferenceException”.

Sorry for my english :S and thanks.

Answer 1

A couple of points here…
1) I would use a Tap event on the item rather than a selection changed event. It helps prevent accidental navigation.
2) If you do use the selectionchanged event, try the following code to get your item. Your current code will attempt to fire if an items is de-selected in the list.

private void allTaskListTasksListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{   
    if(e.AddedItems.Count > 0) // the items that were added to the "selected" collection
    {
        var mySelectedItem = e.AddedItems[0] as myItemType;
        if(null != mySelectedItem) // prevents errors if casting fails
        {
            NavigationService.Navigate(
               new Uri("/View/Details.xaml?msg=" + mySelectedItem.Crm_object_id, 
                       UriKind.RelativeOrAbsolute)
            );
        }
    }
}