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:

代码：

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，我找到了你的问题。当您最初触发WPF应用程序时，它将运行SelectionChanged事件。只要创建了ComboBox对象，就会发生这种情况。问题是，在您的WPF应用程序中，您在RichTextBox之前的XAML中有ComboBox。这意味着此事件在创建RichTextBox之前触发。因此，您将获得Null引用异常。你有两个选择。在尝试对其进行操作之前，您可以吃错误或尝试确定RichTextBox是否存在，或者您可以将XAML中的RichTextBox向上移动到ComboBox上方。这与表单放置无关，而是与XAML中的位置无关。任何一个都可以解决您的问题。

Answer 2

没有更多细节，听起来像EssayContents_richtextbox为空。

Answer 3

或者：

TextAlignment_combobox为空（似乎非常不可能）
EssayContents_richtextbox为空
或EssayContents_richtextbox.Selection为空

你应该调试它并检查所有内容，直到找到null为空的东西，或者以某种方式阻止它们在代码中为null，例如：

private void TextAlignment_combobox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if (EssayContents_richtextbox == null || EssayContents_richtextbox.Selection == null)
    {
        // Handle me, or just
        return;
    }

    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);
    }
}

我猜如果EssayContents_richtextbox为空，这是代码中的错误 - 同时查看Selection属性的文档，看起来这可能会有一个值，即使有没有选择（尽管它没有明确表示它永远不会返回null）

SelectedIndex是一个空引用异常？

3 个答案: