c# - SelectedIndex 是空引用异常？

Question

我一直收到这个错误:

System.NullReferenceException was unhandled by user code
  Message=[Arg_NullReferenceException]
Arguments: 
Debugging resource strings are unavailable. Often the key and arguments provide sufficient information to diagnose the problem. See http://go.microsoft.com/fwlink/?linkid=106663&Version=5.0.60401.00&File=mscorlib.dll&Key=Arg_NullReferenceException
  StackTrace:
       at Jantire.DoHomeworkView.TextAlignment_combobox_SelectionChanged(Object sender, SelectionChangedEventArgs e)
       at System.Windows.Controls.Primitives.Selector.OnSelectionChanged(SelectionChangedEventArgs e)
       at System.Windows.Controls.Primitives.Selector.InvokeSelectionChanged(List`1 unselectedItems, List`1 selectedItems)
       at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
       at System.Windows.Controls.Primitives.Selector.OnItemsChanged(NotifyCollectionChangedEventArgs e)
       at System.Windows.Controls.ItemsControl.OnItemCollectionChanged(Object sender, NotifyCollectionChangedEventArgs e)
       at System.Collections.Specialized.NotifyCollectionChangedEventHandler.Invoke(Object sender, NotifyCollectionChangedEventArgs e)
       at System.Windows.Controls.ItemCollection.NotifyCollectionChanged(NotifyCollectionChangedEventArgs e)
       at System.Windows.Controls.ItemCollection.NotifyCollectionReady()
       at System.Windows.Controls.ItemsControl.NotifyAllItemsAdded(IntPtr nativeItemsControl)
  InnerException: 

在代码处:

private void TextAlignment_combobox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            //This next line is where error is at
            if (TextAlignment_combobox.SelectedIndex == 0)
            {
                EssayContents_richtextbox.Selection.ApplyPropertyValue(Paragraph.TextAlignmentProperty,  TextAlignment.Left);
            }
            if (TextAlignment_combobox.SelectedIndex == 1)
            {
                EssayContents_richtextbox.Selection.ApplyPropertyValue(Paragraph.TextAlignmentProperty, TextAlignment.Center);
            }
            if (TextAlignment_combobox.SelectedIndex == 2)
            {
                EssayContents_richtextbox.Selection.ApplyPropertyValue(Paragraph.TextAlignmentProperty, TextAlignment.Right);
            }
        }

使用 XAML:

<ComboBox Width="128" x:Name="TextAlignment_combobox" SelectionChanged="TextAlignment_combobox_SelectionChanged" ToolTipService.ToolTip="Text Alignment">
                <ComboBoxItem Name="LeftAlignment_comboboxitem" Content="Left Alignment" IsSelected="True"/>
                <ComboBoxItem Name="CenterAlignment_comboboxitem" Content="Center Alignment"/>
                <ComboBoxItem Name="RightAlignment_comboboxitem" Content="Right Alignment"/>
            </ComboBox>

Answer 1

好的，我已经测试了这个场景并且发现了你的问题。当您最初启动 WPF 应用程序时，它会通过 SelectionChanged 事件运行。一旦创建了 ComboBox 对象，就会发生这种情况。问题是，在您的 WPF 应用程序上，XAML 中的组合框位于您的 RichTextBox 之前。这意味着此事件在创建 RichTextBox 之前触发。因此，您会得到一个 Null 引用异常。你有两个选择。您可以吃掉错误或尝试确定 RichTextBox 是否存在，然后再尝试对其进行操作，或者您可以在 XAML 中将 RichTextBox 向上移动到 ComboBox 上方。这与表单放置无关，而是与 XAML 中的放置有关。任何一个都可以解决您的问题。