c# - WPF XAML 绑定(bind)不更新

Question

我有一个 WPF 项目，其中有 4 个 TextBlock。我想要的是通过 Binding 更改每个 TextBlock 的 Text。

到目前为止，我有我的 XAML:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="*"/>
        <RowDefinition Height="*"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <TextBlock x:Name="First" Text="{Binding FirstString}" Grid.Row="0"/>
    <TextBlock x:Name="Second" Text="{Binding SecondString}" Grid.Row="1"/>
    <TextBlock x:Name="Third" Text="{Binding ThirdString}" Grid.Row="2"/>
    <TextBlock x:Name="Fourth" Text="{Binding FourthString}" Grid.Row="3"/>
</Grid>

在我的代码中我有:

public partial class MainWindow : Window
{
    public string FirstString { get; set; }
    public string SecondString { get; set; }
    public string ThirdString { get; set; }
    public string FourthString { get; set; }

    public MainWindow()
    {
        InitializeComponent();

        FirstString = "First";
        SecondString = "Second";
        ThirdString= "Third";
        FourthString= "Fourth";
    }
}

但是 Binding 根本不起作用。我做错了什么吗？

编辑:按照 Chris Mantle 在评论中的建议查看调试输出(我必须为绑定(bind)启用警告)后，我收到以下错误:

System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=FirstString; DataItem=null; target element is 'TextBlock' (Name='First'); target property is 'Text' (type 'String')

Answer 1

有几处是不正确的。 Binding 标记将查看控件的 DataContext 属性中的对象。除非另有说明，否则此属性从声明父级继承 DataContext。对于 Window 控件，这是开箱即用的 null。

这个问题有两种选择。您可以在代码隐藏或 XAML 中显式设置 DataContext

// In XAML
<Window DataContext={Binding RelativeSource={RelativeSource Self}}>

or

// In the code-behind
DataContext = this;

另一个问题是绑定(bind)是在初始化时应用的。最初，您的属性是空的。在 InitializeComponent 阶段之后，控件将“绑定(bind)”到属性(为空)。之后设置属性时，控件无法知道它已更改。有两种机制可以做到这一点。在控件级别，您可以将这些属性设置为 DependencyProperty或实现 INotifyPropertyChanged接口(interface)并提出更改。如果你想走 INPC 路线，你可以这样实现你的属性和窗口:

public partial class MainWindow : INotifyPropertyChanged
{
    private string firstString;
    private string secondString;
    private string thirdString;
    private string fourthString;

    public string FirstString
    {
        get { return firstString; }
        set
        {
            firstString = value;
            RaisePropertyChanged("FirstString");
        }
    }

    public string SecondString
    {
        get { return secondString; }
        set
        {
            secondString = value;
            RaisePropertyChanged("SecondString");
        }
    }

    public string ThirdString
    {
        get { return thirdString; }
        set
        {
            thirdString = value;
            RaisePropertyChanged("ThirdString");
        }
    }

    public string FourthString
    {
        get { return fourthString; }
        set
        {
            fourthString = value;
            RaisePropertyChanged("FourthString");
        }
    }

    public MainWindow()
    {
        DataContext = this;
        InitializeComponent();

        FirstString = "First";
        SecondString = "Second";
        ThirdString = "Third";
        FourthString = "Fourth";
    }

    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    private void RaisePropertyChanged(string propertyName)
    {
        var handlers = PropertyChanged;

        handlers(this, new PropertyChangedEventArgs(propertyName));
    }
}