默认值不能是 'Unset'

Question

我有一个附加的属性当我将其附加到 xaml 元素时我收到一个名为 "Default value cannot be 'Unset'"[=13 的异常=]

我的附件属性逻辑

    public static DataTemplate GetFooterContentTemplate(DependencyObject obj)
    {
        return (DataTemplate)obj.GetValue(FooterContentTemplateProperty);
    }

    public static void SetFooterContentTemplate(DependencyObject obj, DataTemplate value)
    {
        obj.SetValue(FooterContentTemplateProperty, value);
    }
    // Using a DependencyProperty as the backing store for GetFooterContentTemplate.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty FooterContentTemplateProperty =
        DependencyProperty.Register("FooterContentTemplate", typeof(DataTemplate), typeof(RadWindowFooterProperties), new PropertyMetadata(DependencyProperty.UnsetValue));


    public static Object GetFooterContent(DependencyObject obj)
    {
        return (Object)obj.GetValue(FooterContentProperty);
    }

    public static void SetFooterContent(DependencyObject obj, Object value)
    {
        obj.SetValue(FooterContentProperty, value);
    }

    // Using a DependencyProperty as the backing store for RadWindowFooterContent.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty FooterContentProperty =
        DependencyProperty.RegisterAttached("FooterContent", typeof(Object), typeof(RadWindowFooterProperties), new PropertyMetadata(DependencyProperty.UnsetValue));

我的XAML:

     <Grid x:Name="FooterRoot"  Grid.Row="2" MinHeight="42">
                        <Border Background="{StaticResource ShellFooterBrush}"/>
                        <Border Background="{DynamicResource ShellTileBrush}"/>
                        <ContentPresenter Content="{Binding Path=(Local:RadWindowFooterProperties.FooterContent),RelativeSource={RelativeSource Mode=TemplatedParent}}" ContentTemplate="{Binding Path=(Local:RadWindowFooterProperties.FooterContentTemplate),RelativeSource={RelativeSource Mode=TemplatedParent}}" RecognizesAccessKey="True"/>

                    </Grid>

请告诉我哪里做错了。

Answer 1

好吧，如果我读 this 正确，它会指出：

Setting a DefaultValue of UnsetValue is specifically disallowed.

所以我建议设置一个默认值，例如 null 或任何更适合您的用例的值。

Answer 2

DependencyProperty.UnsetValue 不是依赖项属性的有效默认值。除非您想指定 null 以外的默认值，否则根本不需要注册属性元数据。

您还必须更改 FooterContentTemplateProperty 的声明以使用 RegisterAttached 而不是 Register:

public static readonly DependencyProperty FooterContentTemplateProperty =
    DependencyProperty.RegisterAttached(
        "FooterContentTemplate", typeof(DataTemplate), typeof(RadWindowFooterProperties));

public static readonly DependencyProperty FooterContentProperty =
    DependencyProperty.RegisterAttached(
        "FooterContent", typeof(Object), typeof(RadWindowFooterProperties));

默认值不能是 'Unset'

Default value cannot be 'Unset'

c#

wpf

xaml

attached-properties