无法在Setter中设置WPF附加属性，求问题原因

在自定义控件的Style中尝试为Label设置附加属性，移除Setter中的附加属性后程序运行正常，但添加后抛出错误，请问原因是什么？

样式代码

<Style x:Key="DayNumberStyle" TargetType="Label" BasedOn="{StaticResource {x:Type Label}}">
    <Setter Property="HorizontalAlignment" Value="Center"></Setter>
    <Setter Property="local:DateRangePickerHelper.SelectionType" Value="Selected"></Setter>
</Style>

附加属性代码

public class DateRangePickerHelper : FrameworkElement
{
    public static readonly DependencyProperty SelectionTypeProperty = DependencyProperty.RegisterAttached(
        "DateSelectionType", typeof(DateSelectionType), typeof(DateRangePickerHelper),
        new PropertyMetadata(DateSelectionType.NotSelected));

    public static void SetSelectionType(DependencyObject element, DateSelectionType value)
    {
        element.SetValue(SelectionTypeProperty, value);
    }

    public static DateSelectionType GetSelectionType(DependencyObject element)
    {
        return (DateSelectionType)element.GetValue(SelectionTypeProperty);
    }
}

枚举代码

public enum DateSelectionType
{
    NotSelected,
    Selected,
    StartDate,
    EndDate
}

Label代码

<Label Style="{StaticResource DayNumberStyle}" Grid.Column="0" Content="{Binding Sunday}">
   <b:Interaction.Triggers>                                                                                
      <b:EventTrigger EventName="MouseLeftButtonUp">                                                                                    
         <b:InvokeCommandAction Command="{Binding Path=LabelClickedCommand, RelativeSource={RelativeSource AncestorType=local:DateRangePicker}}"                                                                                            CommandParameter="{Binding Sunday}"></b:InvokeCommandAction>                                                                                
      </b:EventTrigger>                                                                             
   </b:Interaction.Triggers>
</Label>

报错信息

Value cannot be null. (Parameter 'property')

原因分析 #

核心问题是附加属性注册名称与Setter引用名称不匹配，违反了WPF附加属性的命名约定：

• 附加属性注册时，RegisterAttached方法的第一个参数是属性的公开名称，这里填的是"DateSelectionType"
• 但Set/Get方法的后缀是SelectionType，Setter里引用的也是local:DateRangePickerHelper.SelectionType
• WPF会根据Setter中的属性名称，自动拼接成GetSelectionType/SetSelectionType方法查找对应附加属性，同时要求注册的属性名必须和方法后缀一致。由于注册名与引用名不匹配，WPF无法找到对应的DependencyProperty实例，最终抛出空引用错误。

修正方案 #

将附加属性的注册名称改为"SelectionType"，与Set/Get方法的后缀保持一致：

public class DateRangePickerHelper : FrameworkElement
{
    // 修正注册名称为"SelectionType"，与方法后缀匹配
    public static readonly DependencyProperty SelectionTypeProperty = DependencyProperty.RegisterAttached(
        "SelectionType", typeof(DateSelectionType), typeof(DateRangePickerHelper),
        new PropertyMetadata(DateSelectionType.NotSelected));

    public static void SetSelectionType(DependencyObject element, DateSelectionType value)
    {
        element.SetValue(SelectionTypeProperty, value);
    }

    public static DateSelectionType GetSelectionType(DependencyObject element)
    {
        return (DateSelectionType)element.GetValue(SelectionTypeProperty);
    }
}

修改后，Style中的Setter就能正确匹配到附加属性，程序可正常运行。

内容的提问来源于stack exchange，提问作者3xGuy