WPF 新手 - 在代码中更新 TextBlock 导致 NullReferenceException
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New to WPF - Updating TextBlock in Code behind causing NullReferenceException
提问by MattSavage
I'm really new to WPF and I'm trying to update the text in a TextBlock whenever the selected item in a ListBox changes.
我真的是 WPF 的新手，每当 ListBox 中的选定项目发生更改时，我都会尝试更新 TextBlock 中的文本。
I added the ListBox and TextBlock to my XAML:
我将 ListBox 和 TextBlock 添加到我的 XAML 中：
<Window x:Class="Blend_Test.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" WindowState="Maximized" ResizeMode="NoResize" Width="{DynamicResource {x:Static SystemParameters.PrimaryScreenWidthKey}}" Height="{DynamicResource {x:Static SystemParameters.PrimaryScreenHeightKey}}">
<Grid Background="#FFC10000">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<ListBox Grid.Column="0" Margin="20" FontSize="48" Name="VideoListBox" SelectedIndex="0" Cursor="None" SelectionChanged="VideoListBox_SelectionChanged">
<ListBoxItem Margin="20">Video 1</ListBoxItem>
<ListBoxItem Margin="20">Video 2</ListBoxItem>
<ListBoxItem Margin="20">Video 3</ListBoxItem>
<ListBoxItem Margin="20">Video 4</ListBoxItem>
</ListBox>
<TextBlock Grid.Column="1" Text="Lorem Ipsum" x:Name="VideoTextBlock" FontSize="48"></TextBlock>
</Grid>
</Window>
But now I'm not exactly sure what to add to my code behind. What I have so far is:
但是现在我不确定要在我的代码后面添加什么。到目前为止我所拥有的是：
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private void VideoListBox_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
{
VideoTextBlock.Text = "Test";
}
}
However when I run this I'm getting a NullReferenceException error. I think I need to initialize the TextBlock somehow, but I'm not sure how to do this.
但是，当我运行它时，我收到了 NullReferenceException 错误。我想我需要以某种方式初始化 TextBlock，但我不确定如何做到这一点。
回答by Dan
Try using a binding rather than an event handler:
尝试使用绑定而不是事件处理程序：
<Window
x:Class="Blend_Test.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow"
WindowState="Maximized"
ResizeMode="NoResize"
Width="{DynamicResource {x:Static SystemParameters.PrimaryScreenWidthKey}}"
Height="{DynamicResource {x:Static SystemParameters.PrimaryScreenHeightKey}}">
<Grid Background="#FFC10000">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<ListBox
Grid.Column="0"
Margin="20"
FontSize="48"
Name="VideoListBox"
SelectedIndex="0"
Cursor="None">
<ListBoxItem Margin="20">Video 1</ListBoxItem>
<ListBoxItem Margin="20">Video 2</ListBoxItem>
<ListBoxItem Margin="20">Video 3</ListBoxItem>
<ListBoxItem Margin="20">Video 4</ListBoxItem>
</ListBox>
<TextBlock
Grid.Column="1"
Text="{Binding SelectedItem.Content, ElementName=VideoListBox}"
x:Name="VideoTextBlock"
FontSize="48"/>
</Grid>
</Window>
If that doesn't work for your needs, I would just check for null before you try to access it:
如果这不能满足您的需求，我只会在您尝试访问它之前检查 null：
private void VideoListBox_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
{
if (VideoTextBlock != null)
{
VideoTextBlock.Text = "Test";
}
}
