I'm learning binding ways in WPF with DataContext, but I cannot understand why it runs like that. Here are the codes below.

Ex1: can get correct binding datas (assign DataContext in code behind)

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

And Ex2: why the Width and the Height number are NaN & Compile failed ?? (use DataContext in 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">
    <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

CodePudding user response：

The expression

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

would create a new MainWindow instance and assign it to the DataContext property.

In order to assign the current instance, you would have to write

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