WPF 因设置不期望的DataContext,导致的绑定异常

2019-12-03 来源: 唐宋元明清2188 发布在  https://www.cnblogs.com/kybs0/p/11977281.html

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

 <Window x:Class="WpfApp8.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:WpfApp8"
         mc:Ignorable="d"
         Title="MainWindow" Height="450" Width="800" x:Name="TheMainWindow">
     <Grid>
         <local:UserControl1 BackgroundTest="{Binding BrushTest}"/>
     </Grid>
 </Window>
     public partial class MainWindow : Window
     {
         public MainWindow()
         {
             InitializeComponent();
             BrushTest = Brushes.Red;
             this.DataContext = this;
         }
         public static readonly DependencyProperty BrushTestProperty = DependencyProperty.Register(
             "BrushTest", typeof(SolidColorBrush), typeof(MainWindow), new PropertyMetadata(default(SolidColorBrush)));
         public SolidColorBrush BrushTest
         {
             get { return (SolidColorBrush) GetValue(BrushTestProperty); }
             set { SetValue(BrushTestProperty, value); }
         }
     }

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。

 <UserControl x:Class="WpfApp8.UserControl1"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:local="clr-namespace:WpfApp8"
              mc:Ignorable="d"
              d:DesignHeight="450" d:DesignWidth="800">
     <Grid Background="{Binding BackgroundTest}">
     </Grid>
 </UserControl>
     public partial class UserControl1 : UserControl
     {
         public UserControl1()
         {
             InitializeComponent();
             this.DataContext = this;
         }
         public static readonly DependencyProperty BackgroundTestProperty = DependencyProperty.Register(
             "BackgroundTest", typeof(SolidColorBrush), typeof(UserControl1), new PropertyMetadata(default(SolidColorBrush)));
         public SolidColorBrush BackgroundTest
         {
             get { return (SolidColorBrush) GetValue(BackgroundTestProperty); }
             set { SetValue(BackgroundTestProperty, value); }
         }
     }

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:

 <UserControl x:Class="WpfApp8.UserControl1"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:local="clr-namespace:WpfApp8"
              mc:Ignorable="d"
              d:DesignHeight="450" d:DesignWidth="800" x:Name="TheUserControl">
     <Grid Background="{Binding ElementName=TheUserControl,Path=BackgroundTest}">
     </Grid>
 </UserControl>

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