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Attached properties overview (WPF .NET)

  • Artikel

An attached property is a Extensible Application Markup Language (XAML) concept. Attached properties enable extra property/value pairs to be set on any XAML element that derives from DependencyObject, 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.

Prerequisites

The article assumes a basic knowledge of dependency properties, and that you've read Dependency properties overview. To follow the examples in this article, it helps if you're familiar with XAML and know how to write Windows Presentation Foundation (WPF) applications.

Why use attached properties

An attached property lets a child element specify a unique value for a property that's defined in a parent element. A common scenario is a child element specifying how it should be rendered in the UI by its parent element. For example, DockPanel.Dock is an attached property because it's set on child elements of a DockPanel, not the DockPanel itself. The DockPanel class defines a static DependencyProperty field, named DockProperty, and then provides GetDock and SetDock methods as public accessors for the attached property.

Attached properties in XAML

In XAML, you set attached properties by using the syntax <attached property provider type>.<property name>, where the attached property provider is the class that defines the attached property. The following example shows how a child element of DockPanel can set the DockPanel.Dock property value.

<DockPanel>
    <TextBox DockPanel.Dock="Top">Enter text</TextBox>
</DockPanel>

The usage is similar to a static property in that you reference the type that owns and registers the attached property (for example, DockPanel), not the instance name.

When you specify an attached property using a XAML attribute, only the set action is applicable. You can't directly get a property value through XAML, although there are some indirect mechanisms for comparing values, such as triggers in styles.

Attached properties in WPF

Attached properties are a XAML concept, dependency properties are a WPF concept. In WPF, most UI-related attached properties on WPF types are implemented as dependency properties. WPF attached properties that are implemented as dependency properties support dependency property concepts, such as property metadata including default values from metadata.

Attached property usage models

Although any object can set an attached property value, that doesn't mean setting a value will produce a tangible result or the value will be used by another object. The main purpose of attached properties is to provide a way for objects from a wide variety of class hierarchies and logical relationships to report common information to the type that defines the attached property. Attached property usage typically follows one of these models:

  • The type that defines the attached property is the parent of the elements that set values for the attached property. The parent type iterates its child objects through internal logic that acts on the object tree structure, obtains the values, and acts on those values in some manner.
  • The type that defines the attached property is used as the child element for various possible parent elements and content models.
  • The type that defines the attached property represents a service. Other types set values for the attached property. Then, when the element that set the property is evaluated in the context of the service, the attached property values are obtained through internal logic of the service class.

An example of a parent-defined attached property

The typical scenario where WPF defines an attached property is when a parent element supports a child element collection, and the parent element implements a behavior based on data reported by each of its child elements.

DockPanel defines the DockPanel.Dock attached property. DockPanel has class-level code, specifically MeasureOverride and ArrangeOverride, that's part of its rendering logic. A DockPanel instance checks whether any of its immediate child elements have set a value for DockPanel.Dock. If so, those values become inputs to the rendering logic applied to each child element. Although it's theoretically possible for attached properties to influence elements beyond the immediate parent, the defined behavior for a nested DockPanel instance is to only interact with its immediate child element collection. So, if you set DockPanel.Dock on an element that has no DockPanel parent, no error or exception will be raised and you would have created a global property value that won't be consumed by any DockPanel.

Attached properties in code

Attached properties in WPF don't have the typical CLR get and set wrapper methods because the properties might be set from outside of the CLR namespace. To permit a XAML processor to set those values when parsing XAML, the class that defines the attached property must implement dedicated accessor methods in the form of Get<property name> and Set<property name>.

You can also use the dedicated accessor methods to get and set an attached property in code, as shown in the following example. In the example, myTextBox is an instance of the TextBox class.

DockPanel myDockPanel = new();
TextBox myTextBox = new();
myTextBox.Text = "Enter text";

// Add child element to the DockPanel.
myDockPanel.Children.Add(myTextBox);

// Set the attached property value.
DockPanel.SetDock(myTextBox, Dock.Top);

Dim myDockPanel As DockPanel = New DockPanel()
Dim myTextBox As TextBox = New TextBox()
myTextBox.Text = "Enter text"

' Add child element to the DockPanel.
myDockPanel.Children.Add(myTextBox)

' Set the attached property value.
DockPanel.SetDock(myTextBox, Dock.Top)

If you don't add myTextBox as a child element of myDockPanel, calling SetDock won't raise an exception or have any effect. Only a DockPanel.Dock value set on a child element of a DockPanel can affect rendering, and the rendering will be the same whether you set the value before or after adding the child element to the DockPanel.

From a code perspective, an attached property is like a backing field that has method accessors instead of property accessors, and can be set on any object without first needing to be defined on those objects.

Attached property metadata

Metadata for an attached property is generally no different than for a dependency property. When registering an attached property, use FrameworkPropertyMetadata to specify characteristics of the property, such as whether the property affects rendering or measurement. When you specify a default value by overriding attached property metadata, that value becomes the default for the implicit attached property on instances of the overriding class. If an attached property value isn't otherwise set, the default value is reported when the property is queried by using the Get<property name> accessor with an instance of the class where you specified the metadata.

To enable property value inheritance on a property, use attached properties instead of non-attached dependency properties. For more information, see Property value inheritance.

Custom attached properties

When to create an attached property

Creating an attached property is useful when:

  • You need a property setting mechanism available to classes other than the defining class. A common scenario is for UI layout, for instance DockPanel.Dock, Panel.ZIndex, and Canvas.Top are all examples of existing layout properties. In the layout scenario, child elements of a layout-controlling element are able to express layout requirements to their layout parent and to set a value for an attached property defined by the parent.

  • One of your classes represents a service, and you want other classes to integrate the service more transparently.

  • You want Visual Studio WPF Designer support, such as the ability to edit a property through the Properties window. For more information, see Control authoring overview.

  • You want to use property value inheritance.

How to create an attached property

If your class defines an attached property solely for use by other types, then your class doesn't need to derive from DependencyObject. Otherwise, follow the WPF model of having an attached property also be a dependency property, by deriving your class from DependencyObject.

Define your attached property as a dependency in the defining class by declaring a public static readonly field of type DependencyProperty. Then, assign the return value of the RegisterAttached method to the field, which is also known as the dependency property identifier. Follow the WPF property naming convention that distinguishes fields from the properties that they represent, by naming the identifier field <property name>Property. Also, provide static Get<property name> and Set<property name> accessor methods, which lets the property system access your attached property.

The following example shows how to register a dependency property using the RegisterAttached method, and how to define the accessor methods. In the example, the name of the attached property is HasFish, so the identifier field is named HasFishProperty, and the accessor methods are named GetHasFish and SetHasFish.

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

The Get accessor

The get accessor method signature is public static object Get<property name>(DependencyObject target), where:

  • target is the DependencyObject from which the attached property is read. The target type can be more specific than DependencyObject. For example, the DockPanel.GetDock accessor method types the target as UIElement because the attached property is intended to be set on UIElement instances. UiElement indirectly derives from DependencyObject.
  • The return type can be more specific than object. For example, the GetDock method types the returned value as Dock because the return value should be a Dock enumeration.

Note

The get accessor for an attached property is required for data binding support in design tools, such as Visual Studio or Blend for Visual Studio.

The Set accessor

The set accessor method signature is public static void Set<property name>(DependencyObject target, object value), where:

  • target is the DependencyObject on which the attached property is written. The target type can be more specific than DependencyObject. For example, the SetDock method types the target as UIElement because the attached property is intended to be set on UIElement instances. UiElement indirectly derives from DependencyObject.
  • The value type can be more specific than object. For example, the SetDock method requires a Dock value. The XAML loader needs to be able to generate the value type from the markup string that represents the attached property value. So, there must be type conversion, value serializer, or markup extension support for the type you use.

Attached property attributes

WPF defines several .NET attributes that provide information about attached properties to reflection processes and also to consumers of reflection and property information, such as designers. Designers use WPF defined .NET attributes to limit the properties shown in the properties window, to avoid overwhelming users with a global list of all attached properties. You might consider applying these attributes to your own custom attached properties. The purpose and syntax of .NET attributes is described in these reference pages:

Learn more

  • For more information about creating an attached property, see Register an attached property.
  • For more advanced usage scenarios for dependency properties and attached properties, see Custom dependency properties.
  • You can register a property as both an attached property and a dependency property, and include conventional property wrappers. In this way, a property can be set on an element by using property wrappers, and also on any other element by using XAML attached property syntax. For an example, see FrameworkElement.FlowDirection.

See also