Question

I am having trouble using the Add method for an ObservableCollection to simply add a new string value to the observablecollection upon a click event. I create my ObservableCollection in a Settings.cs class and then reference that observablecollection throughout multiple pages in my wp7.1 project. This system has worked perfectly for when I need to add several items of one observablecollection to another, either setting one equal to the other or using .Union depending on the purpose needed. In this case though, I am attempting to add a single string item to my ObservableCollection of type string. My code is as follows

Settings.cs

public static Setting<ObservableCollection<string>> Favorites = new Setting<ObservableCollection<string>>("Favorites", null);

Favorites.xaml

<ListBox x:Name="FavoritesListBox" Grid.Row="1" ItemsSource="{Binding}" Margin="12,0,12,0"
             SelectionChanged="FavoritesListBox_SelectionChanged">

FavoritesPage.xaml.cs

protected override void OnNavigatedTo(NavigationEventArgs e)
    {
        string favorUrl = null;
        NavigationContext.QueryString.TryGetValue("curUrl", out favorUrl);
        if (favorUrl != null )
        {
            //This works but the FavoritesListBox items are cleared upon new page navigation or closing
            //this.FavoritesListBox.Items.Add(favorUrl);

            //This does not work!?
            //if (Settings.Favorites.Value == null)
            //{
            //    //Settings.Favorites.Value.Add(favorUrl);
            //}
            //else
            //{
            //    Settings.Favorites.Value.Add(favorUrl);
            //}                     
        }

        //base.OnNavigatedTo(e);
    }

private void FavoritesListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        this.NavigationService.Navigate(new Uri("/MainPage.xaml?favUrl=" + e.AddedItems[0], UriKind.Relative));
    }

using the .Add method in FavoritesPage.xaml.cs does not give me any coding errors but when debugging I get a NullReferenceException. I also tried using .Insert and that did not work either. Please help this seems to be an easy fix but I have not been able to figure this out! Thanks in advance!

Answer 1

You’re referencing a null object after confirming that it is null!

if (Settings.Favorites.Value == null)
{
  Settings.Favorites.Value.Add(favorUrl);  // throws NullReferenceException 
                                           // because Value is null
}

You need to do this:

if (Settings.Favorites.Value == null)
{
  Settings.Favorites.Value = new ObservableCollection<string>();
}
Settings.Favorites.Value.Add(favorUrl);

Alternately, you can change the initialization from

public static Setting<ObservableCollection<string>> Favorites = 
    new Setting<ObservableCollection<string>>("Favorites", null);

to

public static Setting<ObservableCollection<string>> Favorites = 
    new Setting<ObservableCollection<string>>("Favorites", 
      new ObservableCollection<string>());

This way you can avoid the null check.