WPF自定义控件绑定报错：TestValue正常但Inner属性绑定失败

现象描述 #

TestControl的TestValue属性绑定HelloButton的Width可正常工作，但SomePropertyClass的Inner属性进行相同绑定时出现如下错误：

System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=Width; DataItem=null; target element is 'SomePropertyClass' (HashCode=63183526); target property is 'Inner' (type 'Int32')

相关代码 #

1. MainWindow.xaml #

<StackPanel>
    <Button x:Name="HelloButton" Width="100">Hello</Button>
    <lib:TestControl Width="100" TestValue="{Binding ElementName=HelloButton,Path=Width}">
        <lib:TestControl.Some>
            <lib:SomePropertyClass Inner="{Binding ElementName=HelloButton,Path=Width}"/>
        </lib:TestControl.Some>
    </lib:TestControl>
</StackPanel>

2. TestControl.cs #

public class TestControl : Control
{
    static TestControl()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(TestControl), new FrameworkPropertyMetadata(typeof(TestControl)));
    }

    public static readonly DependencyProperty TestValueProperty = DependencyProperty.Register("TestValue", typeof(int), typeof(TestControl));
    public int TestValue
    {
        get => (int)GetValue(TestValueProperty);
        set => SetValue(TestValueProperty, value);
    }

    public static readonly DependencyProperty SomeProperty = DependencyProperty.Register("Some", typeof(SomePropertyClass), typeof(TestControl));
    public SomePropertyClass Some
    {
        get => (SomePropertyClass)GetValue(SomeProperty);
        set => SetValue(SomeProperty, value);
    }
}

public class SomePropertyClass : DependencyObject
{
    public static readonly DependencyProperty InnerProperty = DependencyProperty.Register("Inner", typeof(int), typeof(SomePropertyClass));
    public int Inner
    {
        get => (int)GetValue(InnerProperty);
        set => SetValue(InnerProperty, value);
    }
}

3. TestControl.xaml #

<Style TargetType="{x:Type local:TestControl}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:TestControl}">
                <StackPanel>
                    <Button Content="{TemplateBinding TestValue}"/>
                    <Button Content="{Binding RelativeSource={RelativeSource Mode=TemplatedParent},Path=Some.Inner}"/>
                </StackPanel>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

原因分析 #

WPF中使用ElementName进行绑定时，需要绑定目标所在的对象能够找到主导的FrameworkElement/FrameworkContentElement（即绑定上下文载体），这个载体必须属于WPF的可视化树或逻辑树，具备命名范围解析能力。

• TestControl继承自Control（最终继承FrameworkElement），属于可视化树的一部分，能访问所在的命名范围，因此可以正常解析ElementName=HelloButton的绑定。
• SomePropertyClass仅继承DependencyObject，它没有被纳入可视化树或逻辑树，也没有关联任何FrameworkElement作为上下文，因此无法找到HelloButton对应的命名范围，导致绑定失败。

解决方案 #

最直接的方法是让SomePropertyClass继承Freezable而非DependencyObject。Freezable作为DependencyObject的子类，具备自动关联到所在FrameworkElement上下文的能力，能继承父元素的命名范围和DataContext，从而正常解析ElementName绑定：

public class SomePropertyClass : Freezable
{
    // Freezable必须重写该方法
    protected override Freezable CreateInstanceCore()
    {
        return new SomePropertyClass();
    }

    public static readonly DependencyProperty InnerProperty = DependencyProperty.Register("Inner", typeof(int), typeof(SomePropertyClass));
    public int Inner
    {
        get => (int)GetValue(InnerProperty);
        set => SetValue(InnerProperty, value);
    }
}

修改后，SomePropertyClass的实例会自动关联到TestControl的上下文，就能成功绑定HelloButton的Width属性。

内容的提问来源于stack exchange，提问作者Developer333