在WPF中編輯DataGrid會導致System.NullReferenceException異常

Question

我有一個簡單的應用程序，它從DataBase中讀取專輯列表並填充ListBox（AlbumShowCase）。 每當一個ListBoxItem被選中，我更新一個DataGrid（trackDataGrid）與該專輯中的軌道列表（也來自DataBase）。

問題是，我可以編輯DataGrid中的項目，並且對於所有現有的軌道，更改是持久的。但是，如果我嘗試添加一個新的軌道，一旦我完成編輯行，我得到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; 
} 

例外我ValueConverter後立即發生：

[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 
    } 

的TimeConverter與TimeConverterRule是確保進入軌道長度是有效的配對，併爲所有我知道它的正常工作。 就在用戶編輯發生崩潰的DataGrid的最後一行（空白行）時。而這裏的堆棧跟蹤：

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

我懷疑的是，這是因爲你被綁定到LINQ的結果SQL查詢。當您編輯該行時，WPF會嘗試將新項目「添加」到您的查詢中 - 但它不支持添加。

嘗試是這樣的：

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; 

則可能需要訂閱對電網事件，以便當添加一個項目，你將它添加到的DbContext。

此外，確保AlbumCase是一個公共類，具有公共的，無參數的構造函數。這是因爲WPF會嘗試「新」一個來設置屬性。

Answer 2

public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) 
    { 
//check value null 
if(value==null) return 0; 
     return (int)TimeSpan.Parse((string)value).TotalSeconds; 

    } 

Answer 3

與.net反射戳，這是你正在運行到（在System.Windows.Data.BindingExpression）代碼：

internal override object ConvertProposedValue(object value) 
{ 
    ... 
    Type sourcePropertyType = this.Worker.SourcePropertyType; 
    IValueConverter dynamicConverter = null; 
    CultureInfo culture = base.GetCulture(); 
    if (this.Converter != null) 
    { 
     if (!base.UseDefaultValueConverter) 
     { 
      value = this.Converter.ConvertBack(value, sourcePropertyType, this.ParentBinding.ConverterParameter, culture); 
      if (((value != Binding.DoNothing) && (value != DependencyProperty.UnsetValue)) && !this.IsValidValueForUpdate(value, sourcePropertyType)) 
      { 
       dynamicConverter = this.DynamicConverter; 
      } 
     } 

從我可以理解「SourcePropertyType」是一個空值（然後它會拋出呼叫IsValidValueForUpdate）。

所以，這顯然是一個PresentationFramework中的錯誤（它可以報告一個很好的錯誤並且優雅地失敗），但是它發生的原因是，您傳遞給WPF一個爲null的源屬性類型。也許是因爲通用的開放類型或anonymouse類型。

希望這會有所幫助。

爲了幫助你診斷，我建議你把WPF追溯一個，見SO關於這個問題的主題：How to detect broken WPF Data binding?