Dependency Property Value Precedence

Makale
11/16/2012

This topic explains how the workings of the Windows Presentation Foundation (WPF) property system can affect the value of a dependency property, and describes the precedence by which aspects of the property system apply to the effective value of a property.

This topic contains the following sections.

Prerequisites
The WPF Property System
Dependency Properties Might Be "Set" in Multiple Places
Dependency Property Setting Precedence List
TemplatedParent
The Style Property
Default (Theme) Styles
Dynamic Resource References and Binding
Coercion, Animations, and Base Value
Trigger Behaviors
ClearValue and Value Precedence
Related Topics

Prerequisites

This topic assumes that you understand dependency properties from the perspective of a consumer of existing dependency properties on WPF classes, and have read Dependency Properties Overview. To follow the examples in this topic, you should also understand Extensible Application Markup Language (XAML) and know how to write WPF applications.

The WPF Property System

The WPF property system offers a powerful way to have the value of dependency properties be determined by a variety of factors, enabling features such as real-time property validation, late binding, and notifying related properties of changes to values for other properties. The exact order and logic that is used to determine dependency property values is reasonably complex. Knowing this order will help you avoid unnecessary property setting, and might also clear up confusion over exactly why some attempt to influence or anticipate a dependency property value did not end up resulting in the value you expected.

Dependency Properties Might Be "Set" in Multiple Places

The following is example XAML where the same property (Background) has three different "set" operations that might influence the value.

    <Button Background="Red">
      <Button.Style>
        <Style TargetType="{x:Type Button}">
          <Setter Property="Background" Value="Green"/>
          <Style.Triggers>
            <Trigger Property="IsMouseOver" Value="True">
              <Setter Property="Background" Value="Blue" />
            </Trigger>
          </Style.Triggers>
        </Style>
      </Button.Style>
Click
    </Button>

Here, which color do you expect will apply—red, green, or blue?

With the exception of animated values and coercion, local property sets are set at the highest precedence. If you set a value locally you can expect that the value will be honored, even above any styles or control templates. Here in the example, Background is set to Red locally. Therefore, the style defined in this scope, even though it is an implicit style that would otherwise apply to all elements of that type in that scope, is not the highest precedence for giving the Background property its value. If you removed the local value of Red from that Button instance, then the style would have precedence and the button would obtain the Background value from the style. Within the style, triggers take precedence, so the button will be blue if the mouse is over it, and green otherwise.

Dependency Property Setting Precedence List

The following is the definitive order that the property system uses when assigning the run-time values of dependency properties. Highest precedence is listed first. This list expands on some of the generalizations made in the Dependency Properties Overview.

Property system coercion. For details on coercion, see Coercion, Animation, and Base Value later in this topic.
Active animations, or animations with a Hold behavior. In order to have any practical effect, an animation of a property must be able to have precedence over the base (unanimated) value, even if that value was set locally. For details, see Coercion, Animation, and Base Value later in this topic.
Local value. A local value might be set through the convenience of the "wrapper" property, which also equates to setting as an attribute or property element in XAML, or by a call to the SetValue API using a property of a specific instance. If you set a local value by using a binding or a resource, these each act in the precedence as if a direct value was set.
TemplatedParent template properties. An element has a TemplatedParent if it was created as part of a template (a ControlTemplate or DataTemplate). For details on when this applies, see TemplatedParent later in this topic. Within the template, the following precedence applies:
1. Triggers from the TemplatedParent template.
2. Property sets (typically through XAML attributes) in the TemplatedParent template.
Implicit style. Applies only to the Style property. The Style property is filled by any style resource with a key that matches the type of that element. That style resource must exist either in the page or the application; lookup for an implicit style resource does not proceed into the themes.
Style triggers. The triggers within styles from page or application (these styles might be either explicit or implicit styles, but not from the default styles, which have lower precedence).
Template triggers. Any trigger from a template within a style, or a directly applied template.
Style setters. Values from a Setter within styles from page or application.
Default (theme) style. For details on when this applies, and how theme styles relate to the templates within theme styles, see Default (Theme) Styles later in this topic. Within a default style, the following order of precedence applies:
1. Active triggers in the theme style.
2. Setters in the theme style.
Inheritance. A few dependency properties inherit their values from parent element to child elements, such that they need not be set specifically on each element throughout an application. For details see Property Value Inheritance.
Default value from dependency property metadata. Any given dependency property may have a default value as established by the property system registration of that particular property. Also, derived classes that inherit a dependency property have the option to override that metadata (including the default value) on a per-type basis. See Dependency Property Metadata for more information. Because inheritance is checked before default value, for an inherited property, a parent element default value takes precedence over a child element. Consequently, if an inheritable property is not set anywhere, the default value as specified on the root or parent is used instead of the child element default value.

TemplatedParent

TemplatedParent as a precedence item does not apply to any property of an element that you declare directly in standard application markup. The TemplatedParent concept exists only for child items within a visual tree that come into existence through the application of the template. When the property system searches the TemplatedParent template for a value, it is searching the template that created that element. The property values from the TemplatedParent template generally act as if they were set as a local value on the child element, but this lesser precedence versus the local value exists because the templates are potentially shared. For details, see TemplatedParent.

The Style Property

The order of lookup described earlier applies to all possible dependency properties except one: the Style property. The Style property is unique in that it cannot itself be styled, so the precedence items 5 through 8 do not apply. Also, either animating or coercing Style is not recommended (and animating Style would require a custom animation class). This leaves three ways that the Style property might be set:

Explicit style. The Style property is set directly. In most scenarios, the style is not defined inline, but instead is referenced as a resource, by explicit key. In this case the Style property itself acts as if it were a local value, precedence item 3.
Implicit style. The Style property is not set directly. However, the Style exists at some level in the resource lookup sequence (page, application) and is keyed using a resource key that matches the type the style is to be applied to. In this case, the Style property itself acts by a precedence identified in the sequence as item 5. This condition can be detected by using DependencyPropertyHelper against the Style property and looking for ImplicitStyleReference in the results.
Default style, also known as theme style. The Style property is not set directly, and in fact will read as null up until run time. In this case, the style comes from the run-time theme evaluation that is part of the WPF presentation engine.

For implicit styles not in themes, the type must match exactly—a MyButton Button-derived class will not implicitly use a style for Button.

Default (Theme) Styles

Every control that ships with WPF has a default style. That default style potentially varies by theme, which is why this default style is sometimes referred to as a theme style.

The most important information that is found within a default style for a control is its control template, which exists in the theme style as a setter for its Template property. If there were no template from default styles, a control without a custom template as part of a custom style would have no visual appearance at all. The template from the default style gives the visual appearance of each control a basic structure, and also defines the connections between properties defined in the visual tree of the template and the corresponding control class. Each control exposes a set of properties that can influence the visual appearance of the control without completely replacing the template. For instance, consider the default visual appearance of a Thumb control, which is a component of a ScrollBar.

A Thumb has certain customizable properties. The default template of a Thumb creates a basic stucture / visual tree with several nested Border components to create a bevel look. If a property that is part of the template is intended to be exposed for customization by the Thumb class, then that property must be exposed by a TemplateBinding, within the template. In the case of Thumb, various properties of these borders share a template binding to properties such as Background or BorderThickness. But certain other properties or visual arrangements are hard-coded into the control template or are bound to values that come directly from the theme, and cannot be changed short of replacing the entire template. Generally, if a property comes from a templated parent and is not exposed by a template binding, it cannot be adjusted by styles because there is no easy way to target it. But that property could still be influenced by property value inheritance in the applied template, or by default value.

The theme styles use a type as the key in their definitions. However, when themes are applied to a given element instance, themes lookup for this type is performed by checking the DefaultStyleKey property on a control. This is in contrast to using the literal Type, as implicit styles do. The value of DefaultStyleKey would inherit to derived classes even if the implementer did not change it (the intended way of changing the property is not to override it at the property level, but to instead change its default value in property metadata). This indirection enables base classes to define the theme styles for derived elements that do not otherwise have a style (or more importantly, do not have a template within that style and would thus have no default visual appearance at all). Thus, you can derive MyButton from Button and will still get the Button default template. If you were the control author of MyButton and you wanted a different behavior, you could override the dependency property metadata for DefaultStyleKey on MyButton to return a different key, and then define the relevant theme styles including template for MyButton that you must package with your MyButton control. For more details on themes, styles, and control authoring, see Control Authoring Overview.

Dynamic Resource References and Binding

Dynamic resource references and binding operations are both treated as if they were setting the value to which they are being applied. For instance, a dynamic resource applied to a local value acts per precedence item 3, a binding for a property setter within a theme style applies at precedence item 9, and so on. Because dynamic resource references and binding must both be able to obtain values from the run time state of the application, this entails that the actual process of determining the property value precedence for any given property extends into the run time as well.

Dynamic resource references are not strictly speaking part of the property system, but they do have a lookup order of their own which interacts with the sequence listed above. That precedence is documented more thoroughly in the Resources Overview. The basic summation of that precedence is: element to page root, application, theme, system.

Not only do dynamic resources and bindings operate at the same precedence as a local value, they really are a local value, but with a value that is deferred. One consequence of this is that if you have a dynamic resource or binding in place for a property value, any local value that you set subsequently replaces the dynamic binding or binding entirely. Even if you call ClearValue to clear the locally set value, the dynamic resource or binding will not be restored. In fact, if you call ClearValue on a property that has a dynamic resource or binding in place (with no "literal" local value), they are cleared by the ClearValue call too.

Coercion, Animations, and Base Value

Coercion and animation both act on a value that is termed as the "base value" throughout this SDK. The base value is thus whatever value is determined through evaluating upwards in the items until item 2 is reached.

For an animation, the base value can have an effect on the animated value, if that animation does not specify both "From" and "To" for certain behaviors, or if the animation deliberately reverts to the base value when completed. To see this in practice, run the From, To, and By Animation Target Values Sample. Try setting the local values of the rectangle height in the example, such that the initial local value differs from any "From" in the animation. You will note that the animations start right away using the "From" values and replace the base value once started. The animation might specify to return to the value found before animation once it is completed by specifying the Stop FillBehavior. Afterwards, normal precedence is used for the base value determination.

Multiple animations might be applied to a single property, with each of these animations possibly having been defined from different points in the value precedence. However, these animations will potentially composite their values, rather than just applying the animation from the higher precedence. This depends on exactly how the animations are defined, and the type of the value that is being animated. For more information about animating properties, see Animation Overview.

Coercion applies at the highest level of all. Even an already running animation is subject to value coercion. Certain existing dependency properties in WPF have built-in coercion. For a custom dependency property, you define the coercion behavior for a custom dependency property by writing a CoerceValueCallback and passing the callback as part of metadata when you create the property. You can also override coercion behavior of existing properties by overriding the metadata on that property in a derived class. Coercion interacts with the base value in such a way that the constraints on coercion are applied as those constraints exist at the time, but the base value is still retained. Therefore, if constraints in coercion are later lifted, the coercion will return the closest value possible to that base value, and potentially the coercion influence on a property will cease as soon as all constraints are lifted. For more information about coercion behavior, see Dependency Property Callbacks and Validation.

Trigger Behaviors

Controls often define trigger behaviors as part of their default style in themes. Setting local properties on controls might prevent the triggers from being able to respond to user-driven events either visually or behaviorally. The most common use of a property trigger is for control or state properties such as IsSelected. For instance, by default when a Button is disabled (trigger for IsEnabled is false) then the Foreground value in the theme style is what causes the control to appear "grayed out". But if you have set a local Foreground value, that normal gray-out color will be overruled in precedence by your local property set, even in this property-triggered scenario. Be cautious of setting values for properties that have theme-level trigger behaviors and make sure you are not unduly interfering with the intended user experience for that control.

ClearValue and Value Precedence

The ClearValue method provides an expedient means to clear any locally applied value from a dependency property that is set on an element. However, calling ClearValue is not a guarantee that the default as established in metadata during property registration is the new effective value. All of the other participants in value precedence are still active. Only the locally set value has been removed from the precedence sequence. For instance, if you call ClearValue on a property where that property is also set by a theme style, then the theme value is applied as the new value rather than the metadata-based default. You can see this illustrated in Restoring Default Values Sample, although in this case the applied style is an implicit style defined in resources at the application level. If you want to take all property value participants out of the process and set the value to the registered metadata default, you can obtain that default value definitively by querying the dependency property metadata, and then you can use the default value to locally set the property with a call to SetValue.

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