WPF的新增功能-在導致NullReferenceException的背后更新代碼中的TextBlock

Question

我真的是WPF的新手，並且只要列表框中的選定項發生更改，我就會嘗試更新TextBlock中的文本。

我將ListBox和TextBlock添加到了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>

但是現在我不確定到底要在我的代碼中添加什么。 到目前為止，我有：

    public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void VideoListBox_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
    {
        VideoTextBlock.Text = "Test";
    }
}

但是，當我運行此命令時，出現NullReferenceException錯誤。 我認為我需要以某種方式初始化TextBlock，但是我不確定如何執行此操作。

Answer 1

嘗試使用綁定而不是事件處理程序：

<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>

如果那不能滿足您的需求，我將在您嘗試訪問它之前檢查一下null：

private void VideoListBox_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
{
    if (VideoTextBlock != null)
    {
        VideoTextBlock.Text = "Test";
    }
}