在这个例子中,在 XAML 中使用 DataContext 和隐藏代码有什么区别?

What's the difference in this example between using DataContext in XAML & code behind?

我正在学习 WPF 中与 DataContext 的绑定方式,但我不明白为什么它会这样运行。 以下是代码。

Ex1:可以得到正确的绑定数据(在代码后面赋值DataContext)

Xaml

<Window x:Class="DemoDataContextAndItemSource.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:DemoDataContextAndItemSource"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">

    <StackPanel Margin="15">
        <WrapPanel>
            <TextBlock Text="Window title:  " />
            <TextBox Text="{Binding Title, UpdateSourceTrigger=PropertyChanged}" Width="150" />
        </WrapPanel>
        <WrapPanel Margin="0,10,0,0">
            <TextBlock Text="Window dimensions: " />
            <TextBox Text="{Binding Width}" Width="50" />
            <TextBlock Text=" x " />
            <TextBox Text="{Binding Height}" Width="50" />
        </WrapPanel>
    </StackPanel>
</Window>

cs

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

Compile Success

Ex2:为什么 Width 和 Height 数是 NaN 并且编译失败?? (在 Xaml 中使用 DataContext)

<Window x:Class="DemoDataContextAndItemSource.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:DemoDataContextAndItemSource"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <local:MainWindow/>
    </Window.DataContext>
    <StackPanel Margin="15">
        <WrapPanel>
            <TextBlock Text="Window title:  " />
            <TextBox Text="{Binding Title, UpdateSourceTrigger=PropertyChanged}" Width="150" />
        </WrapPanel>
        <WrapPanel Margin="0,10,0,0">
            <TextBlock Text="Window dimensions: " />
            <TextBox Text="{Binding Width}" Width="50" />
            <TextBlock Text=" x " />
            <TextBox Text="{Binding Height}" Width="50" />
        </WrapPanel>
    </StackPanel>
</Window>

Compile Failed

表达式

<Window.DataContext>
    <local:MainWindow/>
</Window.DataContext>

将创建一个新的 MainWindow 实例并将其分配给 DataContext 属性。

为了分配当前实例,您必须编写

<Window ... DataContext="{Binding RelativeSource={RelativeSource Self}}">