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Base Elements Overview

  • Artikkeli

A high percentage of classes in Windows Presentation Foundation (WPF) are derived from four classes which are commonly referred to in the SDK documentation as the base element classes. These classes are UIElement, FrameworkElement, ContentElement, and FrameworkContentElement. The DependencyObject class is also related, because it is a common base class of both UIElement and ContentElement

Base Element APIs in WPF Classes

Both UIElement and ContentElement are derived from DependencyObject, through somewhat different pathways. The split at this level deals with how a UIElement or ContentElement are used in a user interface and what purpose they serve in an application. UIElement also has Visual in its class hierarchy, which is a class that exposes the lower-level graphics support underlying the Windows Presentation Foundation (WPF). Visual provides a rendering framework by defining independent rectangular screen regions. In practice, UIElement is for elements that will support a larger object model, are intended to render and layout into regions that can be described as rectangular screen regions, and where the content model is deliberately more open, to allow different combinations of elements. ContentElement does not derive from Visual; its model is that a ContentElement would be consumed by something else, such as a reader or viewer that would then interpret the elements and produce the complete Visual for Windows Presentation Foundation (WPF) to consume. Certain UIElement classes are intended to be content hosts: they provide the hosting and rendering for one or more ContentElement classes (DocumentViewer is an example of such a class). ContentElement is used as base class for elements with somewhat smaller object models and that more address the text, information, or document content that might be hosted within a UIElement.

Framework-Level and Core-Level

UIElement serves as the base class for FrameworkElement, and ContentElement serves as the base class for FrameworkContentElement. The reason for this next level of classes is to support a WPF core level that is separate from a WPF framework level, with this division also existing in how the APIs are divided between the PresentationCore and PresentationFramework assemblies. The WPF framework level presents a more complete solution for basic application needs, including the implementation of the layout manager for presentation. The WPF core level provides a way to use much of WPF without taking the overhead of the additional assembly. The distinction between these levels very rarely matters for most typical application development scenarios, and in general you should think of the WPF APIs as a whole and not concern yourself with the difference between WPF framework level and WPF core level. You might need to know about the level distinctions if your application design chooses to replace substantial quantities of WPF framework level functionality, for instance if your overall solution already has its own implementations of user interface (UI) composition and layout.

Choosing Which Element to Derive From

The most practical way to create a custom class that extends WPF is by deriving from one of the WPF classes where you get as much as possible of your desired functionality through the existing class hierarchy. This section lists the functionality that comes with three of the most important element classes to help you decide which class to inherit from.

If you are implementing a control, which is really one of the more common reasons for deriving from a WPF class, you probably want to derive from a class that is a practical control, a control family base class, or at least from the Control base class. For some guidance and practical examples, see Control Authoring Overview.

If you are not creating a control and need to derive from a class that is higher in the hierarchy, the following sections are intended as a guide for what characteristics are defined in each base element class.

If you create a class that derives from DependencyObject, you inherit the following functionality:

  • GetValue and SetValue support, and general property system support.

  • Ability to use dependency properties and attached properties that are implemented as dependency properties.

If you create a class that derives from UIElement, you inherit the following functionality in addition to that provided by DependencyObject:

  • Basic support for animated property values. For more information, see Animation Overview.

  • Basic input event support, and commanding support. For more information, see Input Overview and Commanding Overview.

  • Virtual methods that can be overridden to provide information to a layout system.

If you create a class that derives from FrameworkElement, you inherit the following functionality in addition to that provided by UIElement:

  • Support for styling and storyboards. For more information, see Style and Storyboards Overview.

  • Support for data binding. For more information, see Data Binding Overview.

  • Support for dynamic resource references. For more information, see XAML Resources.

  • Property value inheritance support, and other flags in the metadata that help report conditions about properties to framework services such as data binding, styles, or the framework implementation of layout. For more information, see Framework Property Metadata.

  • The concept of the logical tree. For more information, see Trees in WPF.

  • Support for the practical WPF framework-level implementation of the layout system, including an OnPropertyChanged override that can detect changes to properties that influence layout.

If you create a class that derives from ContentElement, you inherit the following functionality in addition to that provided by DependencyObject:

If you create a class that derives from FrameworkContentElement, you get the following functionality in addition to that provided by ContentElement:

  • Support for styling and storyboards. For more information, see Style and Animation Overview.

  • Support for data binding. For more information, see Data Binding Overview.

  • Support for dynamic resource references. For more information, see XAML Resources.

  • Property value inheritance support, and other flags in the metadata that help report conditions about properties to framework services like data binding, styles, or the framework implementation of layout. For more information, see Framework Property Metadata.

  • You do not inherit access to layout system modifications (such as ArrangeOverride). Layout system implementations are only available on FrameworkElement. However, you inherit an OnPropertyChanged override that can detect changes to properties that influence layout and report these to any content hosts.

Content models are documented for a variety of classes. The content model for a class is one possible factor you should consider if you want to find an appropriate class to derive from. For more information, see WPF Content Model.

Other Base Classes

DispatcherObject

DispatcherObject provides support for the WPF threading model and enables all objects created for WPF applications to be associated with a Dispatcher. Even if you do not derive from UIElement, DependencyObject, or Visual, you should consider deriving from DispatcherObject in order to get this threading model support. For more information, see Threading Model.

Visual

Visual implements the concept of a 2D object that generally requires visual presentation in a roughly rectangular region. The actual rendering of a Visual happens in other classes (it is not self-contained), but the Visual class provides a known type that is used by rendering processes at various levels. Visual implements hit testing, but it does not expose events that report hit-testing positives (these are in UIElement). For more information, see Visual Layer Programming.

Freezable

Freezable simulates immutability in a mutable object by providing the means to generate copies of the object when an immutable object is required or desired for performance reasons. The Freezable type provides a common basis for certain graphics elements such as geometries and brushes, as well as animations. Notably, a Freezable is not a Visual; it can hold properties that become subproperties when the Freezable is applied to fill a property value of another object, and those subproperties might affect rendering. For more information, see Freezable Objects Overview.

Animatable

Animatable is a Freezable derived class that specifically adds the animation control layer and some utility members so that currently animated properties can be distinguished from nonanimated properties.

Control

Control is the intended base class for the type of object that is variously termed a control or component, depending on the technology. In general, WPF control classes are classes that either directly represent a UI control or participate closely in control composition. The primary functionality that Control enables is control templating.

See also