NullPointerException after going back in a NavigationWindow

Question

thats my navigation window

<NavigationWindow x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="800" Width="600" Source="Page1.xaml">

thats my page1

<Page x:Class="WpfApplication1.Page1"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
  mc:Ignorable="d" 
  d:DesignHeight="600" d:DesignWidth="800"
Title="Page1" Name="IndexPage">

<ListView Name="myListView" ItemsSource="{Binding ElementName=IndexPage, Path=SeriesCollection}" ScrollViewer.HorizontalScrollBarVisibility="Disabled" IsSynchronizedWithCurrentItem="True" SelectionChanged="handleSelected">
    <ListView.ItemsPanel >
        <ItemsPanelTemplate>
            <WrapPanel>
            </WrapPanel>
        </ItemsPanelTemplate>            
    </ListView.ItemsPanel>
    <ListView.ItemTemplate>
        <DataTemplate>
            <StackPanel >
                <Image Width="214" Height="317" Source="{Binding Image}"/>
                <Label Content="{Binding Name}" />
            </StackPanel>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

Page 2 is just en empty skeleton

code behind

namespace WpfApplication1
{
/// <summary>
/// Interaktionslogik für Page1.xaml
/// </summary>
public partial class Page1 : Page
{
    private ObservableCollection<Series> _series =
      new ObservableCollection<Series>();

    public ObservableCollection<Series> SeriesCollection
    {
        get { return _series; }
    }

    public Page1()
    {
        InitializeComponent();

        DirectoryInfo baseDir = new DirectoryInfo(@"C:\Serien");
        DirectoryInfo[] dirs = baseDir.GetDirectories();
        foreach (DirectoryInfo dir in dirs)
        {
            Series serie = new Series(dir);
            Console.WriteLine("adding " + serie.Name);
            _series.Add(serie);
        }

        Console.WriteLine(_series.Count);
    }

    public void handleSelected(object sender, RoutedEventArgs args)
    {
        Series currentSerie = (Series) myListView.Items.CurrentItem;

        Page2 page = new Page2();
        this.NavigationService.Navigate(page);

        Console.WriteLine(currentSerie.Name);
        Console.WriteLine(currentSerie.GetType());
        Console.WriteLine(currentSerie.ToString());
    }
}
}

so i click on an item to trigger the SelectionChanged Event to handle it in SelectionChanged where i navigate to page2 , so far so good.

then i use the back button from the navigation window and get stuck with an NullpointerException at

this.NavigationService.Navigate(page);

i dont even know why this method is triggered. So obviosly i am doing something stupid. Pls tell me what it is. Thanks for your time and affort.

Answer 1

The problem here is that you handle the wrong event. I assume that you want to open Page2 by clicking a ListViewItem. Therefore you should use mouse events instead of SelectionChanged.

For example, you can subscribe to StackPanel MouseDown event in your DataTemplate:

<DataTemplate>
    <StackPanel Background="Transparent"
                MouseDown="StackPanel_MouseDown">
        <Image Width="214" Height="317" Source="{Binding Image}"/>
        <Label Content="{Binding Name}"/>
    </StackPanel>
</DataTemplate>

You can access clicked Series using the following:

private void StackPanel_MouseDown(object sender, MouseButtonEventArgs e)
{
    var currentSerie = (Series)((StackPanel)sender).DataContext;
    ...
}

UPD If you need a real click, you may use a trick like this:

<DataTemplate>
    <Button Click="Button_Click">
        <Button.Template>
            <ControlTemplate TargetType="Button">
                <ContentPresenter/>
            </ControlTemplate>
        </Button.Template>
        <StackPanel Background="Transparent">
            <Image Width="214" Height="317" Source="{Binding Image}"/>
            <Label Content="{Binding Name}"/>
        </StackPanel>
    </Button>
</DataTemplate>

We use a Button like a view-model which is able to handle clicks.