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Is SelectedIndex an exception for a NULL link?

I keep getting this error:

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:

in code:

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);
            }
        }

Using 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>
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3 answers

, , . WPF, SelectionChanged. , ComboBox. , WPF ComboBox XAML RichTextBox. , RichTextBox. , Null. . , RichTextBox , , RichTextBox XAML, ComboBox. , - XAML. .

+5

, , EssayContents_richtextbox null.

0

:

  • TextAlignment_combobox null (seems unlikely)
  • EssayContents_richtextbox is null
  • or EssayContents_richtextbox.Selectionnull

You have to debug this and check them all until you find something that is null, or without getting protection from their incorrectness in your code, for example:

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);
    }
}

I would suggest that if it EssayContents_richtextboxis null, this is an error in your code - also looking at the documentation for the property Selection, it seems that it will probably make a difference even if there is no choice (although it does not explicitly say that it will never return null )

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