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DependencyObject Class

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Definition

Namespace:
Windows.UI.Xaml
Edit

Represents an object that participates in the dependency property system. DependencyObject is the immediate base class of many important UI-related classes, such as UIElement, Geometry, FrameworkTemplate, Style, and ResourceDictionary. For more info on how DependencyObject supports dependency properties, see Dependency properties overview.

public ref class DependencyObject
/// [Windows.Foundation.Metadata.ContractVersion(Windows.Foundation.UniversalApiContract, 65536)]
/// [Windows.Foundation.Metadata.MarshalingBehavior(Windows.Foundation.Metadata.MarshalingType.Agile)]
/// [Windows.Foundation.Metadata.Threading(Windows.Foundation.Metadata.ThreadingModel.Both)]
class DependencyObject
[Windows.Foundation.Metadata.ContractVersion(typeof(Windows.Foundation.UniversalApiContract), 65536)]
[Windows.Foundation.Metadata.MarshalingBehavior(Windows.Foundation.Metadata.MarshalingType.Agile)]
[Windows.Foundation.Metadata.Threading(Windows.Foundation.Metadata.ThreadingModel.Both)]
public class DependencyObject
Public Class DependencyObject
Inheritance
Object Platform::Object IInspectable DependencyObject
Derived
Attributes
ContractVersionAttribute MarshalingBehaviorAttribute ThreadingAttribute

Windows requirements

Device family
Windows 10 (introduced in 10.0.10240.0)
API contract
Windows.Foundation.UniversalApiContract (introduced in v1.0)

Examples

This example defines a class that derives from DependencyObject, and defines an attached property along with the identifier field. The scenario for this class is that it is a service class that declares an attached property that other UI elements can set in XAML The service potentially acts on the attached property values on those UI elements at run time.

public abstract class AquariumServices : DependencyObject
{
    public enum Buoyancy {Floats,Sinks,Drifts}

    public static readonly DependencyProperty BuoyancyProperty = DependencyProperty.RegisterAttached(
      "Buoyancy",
      typeof(Buoyancy),
      typeof(AquariumServices),
      new PropertyMetadata(Buoyancy.Floats)
    );
    public static void SetBuoyancy(DependencyObject element, Buoyancy value)
    {
        element.SetValue(BuoyancyProperty, value);
    }
    public static Buoyancy GetBuoyancy(DependencyObject element)
    {
        return (Buoyancy)element.GetValue(BuoyancyProperty);
    }
}

Public Class AquariumServices
    Inherits DependencyObject
    Public Enum Buoyancy
        Floats
        Sinks
        Drifts
    End Enum

    Public Shared ReadOnly BuoyancyProperty As DependencyProperty = _
          DependencyProperty.RegisterAttached(
          "Buoyancy", _
          GetType(Buoyancy), _
          GetType(AquariumServices), _
          New PropertyMetadata(Buoyancy.Floats))


    Public Sub SetBuoyancy(element As DependencyObject, value As Buoyancy)
        element.SetValue(BuoyancyProperty, value)
    End Sub
    Public Function GetBuoyancy(element As DependencyObject) As Buoyancy
        GetBuoyancy = CType(element.GetValue(BuoyancyProperty), Buoyancy)
    End Function
End Class

public static bool ClearSetProperty(DependencyObject targetObject, DependencyProperty targetDP)
{
    if (targetObject == null || targetDP == null)
    {
        throw new ArgumentNullException();
    }
    object localValue = targetObject.ReadLocalValue(targetDP);
    if (localValue == DependencyProperty.UnsetValue)
    {
        return false;
    }
    else
    {
        targetObject.ClearValue(targetDP);
        return true;
    }
}

Public Shared Function ClearSetProperty(targetObject As DependencyObject, targetDP As DependencyProperty) As Boolean
    If targetObject Is Nothing Or targetDP Is Nothing Then
        Throw New ArgumentNullException()
    End If
    Dim localValue As Object = targetObject.ReadLocalValue(targetDP)
    If localValue = DependencyProperty.UnsetValue Then
        ClearSetProperty = False
    Else
        targetObject.ClearValue(targetDP)
        ClearSetProperty = True
    End If
End Function

This example shows a simple dependency property declaration. A call to GetValue constitutes the entirety of the get accessor implementation for the property wrapper of the new dependency property. A call to SetValue constitutes the entirety of the set accessor implementation. For more examples, see Custom dependency properties.

public class Fish : Control
{
    public static readonly DependencyProperty SpeciesProperty =
    DependencyProperty.Register(
    "Species",
    typeof(String),
    typeof(Fish), null
    );
    public string Species
    {
        get { return (string)GetValue(SpeciesProperty); }
        set { SetValue(SpeciesProperty, (string)value); }
    }
}

Public Class Fish
    Inherits Control

    Public Shared ReadOnly SpeciesProperty As DependencyProperty = _
    DependencyProperty.Register(
    "Species", _
    GetType(String), _
    GetType(Fish), _
    Nothing)
    Public Property Species As String
        Get
            Species = CType(GetValue(SpeciesProperty), String)
        End Get
        Set(value As String)
            SetValue(SpeciesProperty, value)
        End Set
    End Property
End Class

Remarks

The DependencyObject class enables dependency property system services on its many derived classes. For more info about the dependency property concept, see Dependency properties overview.

The dependency property system's primary function is to compute the values of properties, and to provide system notification about values that have changed. Another key class that participates in the dependency property system is DependencyProperty. DependencyProperty enables the registration of dependency properties into the property system, whereas DependencyObject as a base class enables objects to use and set the dependency properties.

Here are some notable services and characteristics that DependencyObject provides or supports:

  • Dependency property hosting support for the existing Windows Runtime dependency properties.
  • Custom dependency property hosting support. You register a dependency property by calling the Register method and storing the method's return value as a public static property in your DependencyObject class.
  • Attached property hosting support for the existing Windows Runtime attached properties.
  • Custom attached property hosting support. You register a dependency property for the attached property usage by calling the RegisterAttached method and storing the method's return value as a public static property in your class.
  • Get and Set utility methods for values of any dependency properties that exist on a DependencyObject. You use these when defining custom dependency property "wrappers" and can also use them from app code as an alternative to using existing "wrapper" properties.
  • Advanced-scenario utility for examining metadata or property values (for example GetAnimationBaseValue).
  • Enforcement of thread affinity to the main UI thread of the Windows Runtime for all DependencyObject instances.
  • The Dispatcher property for advanced threading scenarios. Getting the Dispatcher value provides a reference to a CoreDispatcher object. With the CoreDispatcher, a worker thread can run code that use a DependencyObject but is not on the UI thread, because the CoreDispatcher can defer the execution to an asynchronous operation that won't block or otherwise interfere with the UI thread. See "DependencyObject and threading" section below.
  • Basic data binding and styling support, by enabling properties to be set as expressions to be evaluated at some later point in an object's lifetime. These concepts are explained in more detail in Dependency properties overview. See also Data binding in depth.

DependencyObject and threading

All DependencyObject instances must be created on the UI thread that is associated with the current Window for an app. This is enforced by the system, and there are two important implications of this for your code:

  • Code that uses API from two DependencyObject instances will always be run on the same thread, which is always the UI thread. You don't typically run into threading issues in this scenario.
  • Code that is not running on the main UI thread cannot access a DependencyObject directly because a DependencyObject has thread affinity to the UI thread only. Only code that runs on the UI thread can change or even read the value of a dependency property. For example a worker thread that you've initiated with a .NET Task or an explicit ThreadPool thread won't be able to read dependency properties or call other APIs.

You aren't completely blocked from using a DependencyObject from a worker thread. But you must get a CoreDispatcher object (the value of DependencyObject.Dispatcher) from a DependencyObject in order to get across the deliberate separation between the app UI thread and any other threads running on the system. The CoreDispatcher exposes the RunAsync method. Call RunAsync to run your awaitable code (an IAsyncAction). If it's simple code you can use a lambda expression, otherwise you can implement as a delegate (DispatchedHandler). The system determines a time that your code can be run. Because it's enabling access across threads, DependencyObject.Dispatcher is the only instance API of DependencyObject or any of its subclasses that can be accessed from a non-UI thread without throwing a cross-thread exception. All other DependencyObject APIs throw an exception if you attempt to call them from a worker thread or any other non-UI thread.

Threading issues can usually be avoided in typical UI code. However, devices aren't usually associated with the UI thread. If you are using info obtained from a device to update the UI in real-time, you often must get a CoreDispatcher so that you can update the UI. Services are another case where the code you use to access the service might not be running on the UI thread.

One code scenario where you might encounter DependencyObject-related threading issues if you are defining your own DependencyObject types and you attempt to use them for data sources, or other scenarios where a DependencyObject isn't necessarily appropriate (because the object is not directly related to the UI). For example, you might be attempting perf optimizations with background threads or other worker threads that are changing values of the objects prior to presentation, or in response to a device, service or other external input. Evaluate whether you really need dependency properties for your scenario; maybe standard properties are adequate.

DependencyObject derived classes

DependencyObject is the parent class for several immediately derived classes that are all fundamental to the programming model you use for your app and its XAML UI. Here are some of the notable derived classes:

Constructors

DependencyObject()

Provides base class initialization behavior for DependencyObject derived classes.

Properties

Dispatcher

Gets the CoreDispatcher that this object is associated with. The CoreDispatcher represents a facility that can access the DependencyObject on the UI thread even if the code is initiated by a non-UI thread.

Methods

ClearValue(DependencyProperty)

Clears the local value of a dependency property.
GetAnimationBaseValue(DependencyProperty)

Returns any base value established for a dependency property, which would apply in cases where an animation is not active.
GetValue(DependencyProperty)

Returns the current effective value of a dependency property from a DependencyObject.
ReadLocalValue(DependencyProperty)

Returns the local value of a dependency property, if a local value is set.
RegisterPropertyChangedCallback(DependencyProperty, DependencyPropertyChangedCallback)

Registers a notification function for listening to changes to a specific DependencyProperty on this DependencyObject instance.
SetValue(DependencyProperty, Object)

Sets the local value of a dependency property on a DependencyObject.
UnregisterPropertyChangedCallback(DependencyProperty, Int64)

Cancels a change notification that was previously registered by calling RegisterPropertyChangedCallback.

Applies to

See also