在MainWindow中,创建一个背景属性BrushTest,并将其绑定至界面上UserControl的BackgroundTest属性
1 <Window x:Class="WpfApp8.MainWindow"
2 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
3 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
4 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
5 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
6 xmlns:local="clr-namespace:WpfApp8"
7 mc:Ignorable="d"
8 Title="MainWindow" Height="450" Width="800" x:Name="TheMainWindow">
9 <Grid>
10 <local:UserControl1 BackgroundTest="{Binding BrushTest}"/>
11 </Grid>
12 </Window>
1 public partial class MainWindow : Window
2 {
3 public MainWindow()
4 {
5 InitializeComponent();
6 BrushTest = Brushes.Red;
7 this.DataContext = this;
8 }
10 public static readonly DependencyProperty BrushTestProperty = DependencyProperty.Register(
11 "BrushTest", typeof(SolidColorBrush), typeof(MainWindow), new PropertyMetadata(default(SolidColorBrush)));
13 public SolidColorBrush BrushTest
14 {
15 get { return (SolidColorBrush) GetValue(BrushTestProperty); }
16 set { SetValue(BrushTestProperty, value); }
17 }
18 }
UserControl,同样添加一个BackgroundTest属性,并将其绑定至界面。
1 <UserControl x:Class="WpfApp8.UserControl1"
2 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
3 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
4 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
5 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
6 xmlns:local="clr-namespace:WpfApp8"
7 mc:Ignorable="d"
8 d:DesignHeight="450" d:DesignWidth="800">
9 <Grid Background="{Binding BackgroundTest}">
10 </Grid>
11 </UserControl>
1 public partial class UserControl1 : UserControl
2 {
3 public UserControl1()
4 {
5 InitializeComponent();
6 this.DataContext = this;
7 }
8 public static readonly DependencyProperty BackgroundTestProperty = DependencyProperty.Register(
9 "BackgroundTest", typeof(SolidColorBrush), typeof(UserControl1), new PropertyMetadata(default(SolidColorBrush)));
10 public SolidColorBrush BackgroundTest
11 {
12 get { return (SolidColorBrush) GetValue(BackgroundTestProperty); }
13 set { SetValue(BackgroundTestProperty, value); }
14 }
15 }
运行后,控制台输出绑定异常,背景设置并没有生效。
System.Windows.Data Error: 40 : BindingExpression path error: 'BrushTest' property not found on 'object' ''UserControl1' (Name='')'.
BindingExpression:Path=BrushTest; DataItem='UserControl1' (Name=''); target element is 'UserControl1' (Name=''); target property is 'BackgroundTest' (type 'SolidColorBrush')
为何错了?
因为UserControl设置了俩次DataContext,UserControl1内部设置的上下文覆盖了主窗口设置的上下文。
窗口内<local:UserControl1 BackgroundTest="{Binding BrushTest}"/>绑定的值BrushTest,在UserControl下的上下文无法找到相关值,所以报错了
此类绑定异常,一不小心还是很容易出现的。
在窗口设置了DataContext时(自身或者ViewModel),子控件也设置DataContext。有趣的是,在Xaml编辑时,使用Reshaper链接到的是窗口所在的上下文属性。
所以,子控件设置DataContext时,需要关注下是否有属性被引用绑定外界数据。
建议子控件减少DataContext的使用,以上可以通过指定数据源进行绑定。比如:
1 <UserControl x:Class="WpfApp8.UserControl1"
2 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
3 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
4 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
5 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
6 xmlns:local="clr-namespace:WpfApp8"
7 mc:Ignorable="d"
8 d:DesignHeight="450" d:DesignWidth="800" x:Name="TheUserControl">
9 <Grid Background="{Binding ElementName=TheUserControl,Path=BackgroundTest}">
10 </Grid>
11 </UserControl>
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