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WPF 因设置不期望的DataContext,导致的绑定异常

在MainWindow中,创建一个背景属性BrushTest,并将其绑定至界面

 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     }

并在窗口中添加一个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')

为何错了呢?

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>

 

本文转载自博客园,原文链接:https://www.cnblogs.com/kybs0/p/11977281.html

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