为什么无法显示 DependencyProperty 的默认值

Question

我希望文本块显示附加依赖属性的默认值，但它不起作用。

在xml中

<TextBlock Text="{Binding RelativeSource={RelativeSource Self},Path=MyData}"></TextBlock>

依赖属性

public class MyDependencyObject
{
    public static readonly DependencyProperty MyDataProperty =
        DependencyProperty.RegisterAttached("MyData", typeof(string), typeof(TextBlock),
            new FrameworkPropertyMetadata("MyDependencyObject"));


    public static string GetMyData(DependencyObject dpo)
    {
        return (string)dpo.GetValue(MyDataProperty);
    }

    public static void SetMyData(DependencyObject dpo, string value)
    {
        dpo.SetValue(MyDataProperty, value);
    }
}

当我设置 MyData 的值时，它运行良好。

   <TextBlock local:MyDependencyObject.MyData="Test" Text="{Binding RelativeSource={RelativeSource Self}, Path=MyData}" />

Answer 1

绑定到附加属性的数据需要括号中的Path ，请参阅PropertyPath for Objects in Data Binding ：

<TextBlock Text="{Binding RelativeSource={RelativeSource Self},
                          Path=(local:MyDependencyObject.MyData)}"/>

附加属性声明必须使用声明类作为RegisterAttached的ownerType参数

public class MyDependencyObject
{
    public static readonly DependencyProperty MyDataProperty =
        DependencyProperty.RegisterAttached(
            "MyData",
            typeof(string),
            typeof(MyDependencyObject), // here
            new FrameworkPropertyMetadata("MyDependencyObject"));

    public static string GetMyData(DependencyObject obj)
    {
        return (string)obj.GetValue(MyDataProperty);
    }

    public static void SetMyData(DependencyObject obj, string value)
    {
        obj.SetValue(MyDataProperty, value);
    }
}

尽管从未在 TextBlock 上显式设置附加属性，但它将显示“MyDependencyObject”默认值。