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dependencyobject.md

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-api-id -api-type
T:Microsoft.UI.Xaml.DependencyObject
winrt class

Microsoft.UI.Xaml.DependencyObject

-description

Represents an object that participates in the dependency property system.

-remarks

The DependencyObject class enables dependency property system services on its many derived classes, and 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.

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 DispatcherQueue property for advanced threading scenarios. The DispatcherQueue lets a worker thread run code that uses a DependencyObject but is not on the UI thread, because it 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 DispatcherQueue object (the value of DependencyObject.DispatcherQueue) 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 DispatcherQueue exposes the TryEnqueue method to run your awaitable code. Because it's enabling access across threads, DependencyObject.DispatcherQueue 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 DispatcherQueue 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:

-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.

[!code-csharpDOMain]

[!code-csharpDOCheckClear]

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.

[!code-csharpDOSimpleDP]

-see-also

DependencyProperty, Dependency properties overview, Custom dependency properties, Custom attached properties, Attached properties overview