Unable to set Attached Property in a Setter

Question

I have a Style in a custom control, in which I am trying to set an attached property on a Label.

The Style

<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>

The Attached Property

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);
    }
}

The Enum

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

The Label

<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>

The Error

Value cannot be null. (Parameter 'property')

When I remove the attached property from the Setter everything works correctly.

Can someone explain why I cannot set this with the Style Setter?

Answer 1

You define a dependency property SelectionTypeProperty, so its name must be SelectionType, as your can read in the documentation: How to implement a dependency property (WPF .NET)

The identifier field must follow the naming convention <property name>Property. For instance, if you register a dependency property with the name Location, then the identifier field should be named LocationProperty.

However, in your definition, you pass DateSelectionType name, which is the name of the type of the property, but not the name of the property itself.

If you fail to follow this naming pattern, then WPF designers might not report your property correctly, and aspects of the property system style application might not behave as expected.

Change the name to SelectionType and it works as expected.

public static readonly DependencyProperty SelectionTypeProperty = DependencyProperty.RegisterAttached(
     "SelectionType", typeof(DateSelectionType), typeof(DateRangePickerHelper),
     new PropertyMetadata(DateSelectionType.NotSelected));

In case you ever implement a regular dependency property that has property wrappers instead of methods, you could use nameof(<YourProperty>) to prevent this and renaming issues.