How to register an attached property (WPF .NET)

This article describes how to register an attached property and provide public accessors that let you access the attached property through Extensible Application Markup Language (XAML) and code. Attached properties enable extra property/value pairs to be set on any XAML element, even though the element doesn't define those extra properties in its object model. The extra properties are globally accessible. Attached properties are typically defined as a specialized form of dependency property that doesn't have a conventional property wrapper. Most attached properties for Windows Presentation Foundation (WPF) types are also implemented as dependency properties. You can create dependency properties on any DependencyObject derived type.

Important

The Desktop Guide documentation for .NET 7 and .NET 6 is under construction.

Example

The following example shows how to register an attached property as a dependency property, by using the RegisterAttached method. The provider class has the option of specifying a default value in property metadata. For more information on property metadata, see Dependency property metadata. In this example, the HasFish property has a Boolean value type, with its default value set to false.

The provider class for an attached property must provide static get/set accessor methods that follow the naming convention Get<property name> and Set<property name>. The XAML reader uses the accessors to recognize the XAML attribute for the attached property and resolve its value to the appropriate type. These accessors are necessary even if an attached property isn't registered as a dependency property.

public class Aquarium : UIElement
{
    // Register an attached dependency property with the specified
    // property name, property type, owner type, and property metadata.
    public static readonly DependencyProperty HasFishProperty =
        DependencyProperty.RegisterAttached(
          "HasFish",
          typeof(bool),
          typeof(Aquarium),
          new FrameworkPropertyMetadata(defaultValue: false,
              flags: FrameworkPropertyMetadataOptions.AffectsRender)
        );

    // Declare a get accessor method.
    public static bool GetHasFish(UIElement target) =>
        (bool)target.GetValue(HasFishProperty);

    // Declare a set accessor method.
    public static void SetHasFish(UIElement target, bool value) =>
        target.SetValue(HasFishProperty, value);
}
Public Class Aquarium
    Inherits UIElement

    ' Register an attached dependency property with the specified
    ' property name, property type, owner type, and property metadata.
    Public Shared ReadOnly HasFishProperty As DependencyProperty =
        DependencyProperty.RegisterAttached("HasFish", GetType(Boolean), GetType(Aquarium),
            New FrameworkPropertyMetadata(defaultValue:=False,
                flags:=FrameworkPropertyMetadataOptions.AffectsRender))

    ' Declare a get accessor method.
    Public Shared Function GetHasFish(target As UIElement) As Boolean
        Return target.GetValue(HasFishProperty)
    End Function

    ' Declare a set accessor method.
    Public Shared Sub SetHasFish(target As UIElement, value As Boolean)
        target.SetValue(HasFishProperty, value)
    End Sub

End Class

See also