Question

I have a simple application that reads a list of Albums from a DataBase and fills a ListBox (AlbumShowCase).
Whenever a ListBoxItem is selected I update a DataGrid (trackDataGrid) with the list of tracks in that Album (also from the DataBase).

The problem is, I can edit the items in the DataGrid, and for all the existing tracks, changes are persistent. But if I try to add a new track, once I finish editing the row I get the System.NullReferenceException.

private TunesDBDataContext db;

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    db = new TunesDBDataContext("TunesDB.sdf");
    var query = from a in db.Albums select new AlbumCase(a);
    AlbumShowCase.ItemsSource = query;
}

private void trackDataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    db.SubmitChanges();
}

private void ListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    var query = from a in db.Albums
                where a.AlbumID == ((AlbumCase)e.AddedItems[0]).Album.AlbumID
                select a.Tracks;

    trackDataGrid.ItemsSource = query;
}

The Exception occurs right after my ValueConverter:

[ValueConversion(typeof(String), typeof(int))]
public class TimeConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        int time = (int)value;
        TimeSpan ts = TimeSpan.FromSeconds(time);
        return ts.ToString();
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        // The validation runs before this, so we know that if we got here
        // the data must be valid and won't throw an exception.
        return (int)TimeSpan.Parse((string)value).TotalSeconds;
        // THE EXCEPTION OCCURS AFTER THIS LINE FOR NEW ROWS
    }

The TimeConverter is paired with a TimeConverterRule that makes sure the track length entered is valid, and for all I know it’s working fine.
It’s just when the user edits the LAST row (the empty one) of the DataGrid that the crash occurs. And here’s the stack trace:

System.NullReferenceException was unhandled
Message=Object reference not set to an instance of an object.
Source=PresentationFramework
StackTrace:
    at System.Windows.Data.BindingExpression.IsValidValueForUpdate(Object value, Type sourceType)
    at System.Windows.Data.BindingExpression.ConvertProposedValue(Object value)
    at System.Windows.Data.BindingExpression.ValidateAndConvertProposedValue(Collection1& values)
    at System.Windows.Controls.DataGridHelper.ValidateWithoutUpdate(FrameworkElement element)
    at System.Windows.Controls.DataGridColumn.CommitCellEdit(FrameworkElement editingElement)
    at System.Windows.Controls.DataGridColumn.CommitEdit(FrameworkElement editingElement)
    at System.Windows.Controls.DataGridCell.CommitEdit()
    at System.Windows.Controls.DataGrid.OnExecutedCommitEdit(ExecutedRoutedEventArgs e)
    at System.Windows.Controls.DataGrid.OnExecutedCommitEdit(Object sender, ExecutedRoutedEventArgs e)
    at System.Windows.Input.CommandBinding.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
    at System.Windows.Input.CommandManager.ExecuteCommandBinding(Object sender, ExecutedRoutedEventArgs e, CommandBinding commandBinding)
    at System.Windows.Input.CommandManager.FindCommandBinding(CommandBindingCollection commandBindings, Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
    at System.Windows.Input.CommandManager.FindCommandBinding(Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
    at System.Windows.Input.CommandManager.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
    at System.Windows.UIElement.OnExecutedThunk(Object sender, ExecutedRoutedEventArgs e)
    at System.Windows.Input.ExecutedRoutedEventArgs.InvokeEventHandler(Delegate genericHandler, Object target)
    at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
    at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
    at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
    at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
    at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
    at System.Windows.Input.RoutedCommand.ExecuteImpl(Object parameter, IInputElement target, Boolean userInitiated)
    at System.Windows.Input.RoutedCommand.Execute(Object parameter, IInputElement target)
    at System.Windows.Controls.DataGrid.EndEdit(RoutedCommand command, DataGridCell cellContainer, DataGridEditingUnit editingUnit, Boolean exitEditMode)
    at System.Windows.Controls.DataGrid.CommitAnyEdit()
    at System.Windows.Controls.DataGrid.OnEnterKeyDown(KeyEventArgs e)
    at System.Windows.Controls.DataGrid.OnKeyDown(KeyEventArgs e)
    etc...etc...
    }

Answer 1

My suspicion is that this is because you are binding to the results of a LINQ to SQL query. When you edit the row, WPF tries to “Add” the new item to your query – but it doesn’t support adding.

Try something like this:

var query = from a in db.Albums
            where a.AlbumID == ((AlbumCase)e.AddedItems[0]).Album.AlbumID
            select a.Tracks;

var dataSource = new ObservableCollection<Track>();
foreach (var item in query)
    dataSource.Add(item);

trackDataGrid.ItemsSource = dataSource;

You might then need to subscribe to events on the grid so that when an item is added, you add it to the DbContext.

Also, make sure AlbumCase is a public class with a public, parameterless constructor. That’s because WPF will try to “new” one up to set properties on.