# Styling and Templating

Windows Presentation Foundation (WPF) styling and templating refer to a suite of features (styles, templates, triggers, and storyboards) that allow developers and designers to create visually compelling effects and to create a consistent appearance for their product. Although developers and or designers can customize the appearance extensively on an application-by-application basis, a strong styling and templating model is necessary to allow maintenance and sharing of the appearance within and among applications. Windows Presentation Foundation (WPF) provides that model.

Another feature of the WPF styling model is the separation of presentation and logic. This means that designers can work on the appearance of an application by using only XAML at the same time that developers work on the programming logic by using C# or Visual Basic.

This overview focuses on the styling and templating aspects of the application and does not discuss any data binding concepts. For information about data binding, see Data Binding Overview.

In addition, it is important to understand resources, which are what enable styles and templates to be reused. For more information about resources, see XAML Resources.

## Styling and Templating Sample

The code examples used in this overview are based on a simple photo sample shown in the following illustration:

This simple photo sample uses styling and templating to create a visually compelling user experience. The sample has two TextBlock elements and a ListBox control that is bound to a list of images. For the complete sample, see Introduction to Styling and Templating Sample.

## Style Basics

You can think of a Style as a convenient way to apply a set of property values to more than one element. For example, consider the following TextBlock elements and their default appearance:

<TextBlock>My Pictures</TextBlock>
<TextBlock>Check out my new pictures!</TextBlock>


You can change the default appearance by setting properties, such as FontSize and FontFamily, on each TextBlock element directly. However, if you want your TextBlock elements to share some properties, you can create a Style in the Resources section of your XAML file, as shown here:

<Window.Resources>

<!--A Style that affects all TextBlocks-->
<Style TargetType="TextBlock">
<Setter Property="HorizontalAlignment" Value="Center" />
<Setter Property="FontFamily" Value="Comic Sans MS"/>
<Setter Property="FontSize" Value="14"/>
</Style>

</Window.Resources>


When you set the TargetType of your style to the TextBlock type, the style is applied to all the TextBlock elements in the window.

Now the TextBlock elements appear as follows:

### Extending Styles

Perhaps you want your two TextBlock elements to share some property values, such as the FontFamily and the centered HorizontalAlignment, but you also want the text "My Pictures" to have some additional properties. You can do that by creating a new style that is based on the first style, as shown here:

<Window.Resources>

<!--A Style that extends the previous TextBlock Style-->
<!--This is a "named style" with an x:Key of TitleText-->
<Style BasedOn="{StaticResource {x:Type TextBlock}}"
TargetType="TextBlock"
x:Key="TitleText">
<Setter Property="FontSize" Value="26"/>
<Setter Property="Foreground">
<Setter.Value>
</Setter.Value>
</Setter>
</Style>

</Window.Resources>


Notice that the previous style is given an x:Key. To apply the style, you set the Style property on your TextBlock to the x:Key value, as shown here:

<TextBlock Style="{StaticResource TitleText}" Name="textblock1">My Pictures</TextBlock>
<TextBlock>Check out my new pictures!</TextBlock>


This TextBlock style now has a HorizontalAlignment value of Center, a FontFamily value of Comic Sans MS, a FontSize value of 26, and a Foreground value set to the LinearGradientBrush shown in the example. Notice that it overrides the FontSize value of the base style. If there is more than one Setter setting the same property in a Style, the Setter that is declared last takes precedence.

The following shows what the TextBlock elements now look like:

This TitleText style extends the style that has been created for the TextBlock type. You can also extend a style that has an x:Key by using the x:Key value. For an example, see the example provided for the BasedOn property.

### Relationship of the TargetType Property and the x:Key Attribute

As shown in the first example, setting the TargetType property to TextBlock without assigning the style an x:Key causes the style to be applied to all TextBlock elements. In this case, the x:Key is implicitly set to {x:Type TextBlock}. This means that if you explicitly set the x:Key value to anything other than {x:Type TextBlock}, the Style is not applied to all TextBlock elements automatically. Instead, you must apply the style (by using the x:Key value) to the TextBlock elements explicitly. If your style is in the resources section and you do not set the TargetType property on your style, then you must provide an x:Key.

In addition to providing a default value for the x:Key, the TargetType property specifies the type to which setter properties apply. If you do not specify a TargetType, you must qualify the properties in your Setter objects with a class name by using the syntax Property="ClassName.Property". For example, instead of setting Property="FontSize", you must set Property to "TextBlock.FontSize" or "Control.FontSize".

Also note that many WPF controls consist of a combination of other WPF controls. If you create a style that applies to all controls of a type, you might get unexpected results. For example, if you create a style that targets the TextBlock type in a Window, the style is applied to all TextBlock controls in the window, even if the TextBlock is part of another control, such as a ListBox.

### Styles and Resources

You can use a style on any element that derives from FrameworkElement or FrameworkContentElement. The most common way to declare a style is as a resource in the Resources section in a XAML file, as shown in the previous examples. Because styles are resources, they obey the same scoping rules that apply to all resources; where you declare a style affects where the style can be applied. For example, if you declare the style in the root element of your application definition XAML file, the style can be used anywhere in your application. If you create a navigation application and declare the style in one of the application's XAML files, the style can be used only in that XAML file. For more information about scoping rules for resources, see XAML Resources.

### Setting Styles Programmatically

To assign a named style to an element programmatically, get the style from the resources collection and assign it to the element's Style property. Note that the items in a resources collection are of type Object. Therefore, you must cast the retrieved style to a Style before assigning it to the Style property. For example, to set the defined TitleText style on a TextBlock named textblock1, do the following:

textblock1.Style = (Style)(this.Resources["TitleText"]);

textblock1.Style = CType(Me.Resources("TitleText"), Style)


Note that once a style has been applied, it is sealed and cannot be changed. If you want to dynamically change a style that has already been applied, you must create a new style to replace the existing one. For more information, see the IsSealed property.

You can create an object that chooses a style to apply based on custom logic. For an example, see the example provided for the StyleSelector class.

### Bindings, Dynamic Resources, and Event Handlers

Note that you can use the Setter.Value property to specify a Binding Markup Extension or a DynamicResource Markup Extension. For more information, see the examples provided for the Setter.Value property.

So far, this overview only discusses the use of setters to set property value. You can also specify event handlers in a style. For more information, see EventSetter.

## Data Templates

In this sample application, there is a ListBox control that is bound to a list of photos:

<ListBox ItemsSource="{Binding Source={StaticResource MyPhotos}}"
Background="Silver" Width="600" Margin="10" SelectedIndex="0"/>


This ListBox currently looks like the following:

Most controls have some type of content, and that content often comes from data that you are binding to. In this sample, the data is the list of photos. In WPF, you use a DataTemplate to define the visual representation of data. Basically, what you put into a DataTemplate determines what the data looks like in the rendered application.

In our sample application, each custom Photo object has a Source property of type string that specifies the file path of the image. Currently, the photo objects appear as file paths.

For the photos to appear as images, you create a DataTemplate as a resource:

<Window.Resources>

<!--DataTemplate to display Photos as images
instead of text strings of Paths-->
<DataTemplate DataType="{x:Type local:Photo}">
<Border Margin="3">
<Image Source="{Binding Source}"/>
</Border>
</DataTemplate>

</Window.Resources>


Notice that the DataType property is very similar to the TargetType property of the Style. If your DataTemplate is in the resources section, when you specify the DataType property to a type and not assign it an x:Key, the DataTemplate is applied whenever that type appears. You always have the option to assign the DataTemplate with an x:Key and then set it as a StaticResource for properties that take DataTemplate types, such as the ItemTemplate property or the ContentTemplate property.

Essentially, the DataTemplate in the above example defines that whenever there is a Photo object, it should appear as an Image within a Border. With this DataTemplate, our application now looks like this:

The data templating model provides other features. For example, if you are displaying collection data that contains other collections using a HeaderedItemsControl type such as a Menu or a TreeView, there is the HierarchicalDataTemplate. Another data templating feature is the DataTemplateSelector, which allows you to choose a DataTemplate to use based on custom logic. For more information, see Data Templating Overview, which provides a more in-depth discussion of the different data templating features.

## Control Templates

In WPF, the ControlTemplate of a control defines the appearance of the control. You can change the structure and appearance of a control by defining a new ControlTemplate for the control. In many cases, this gives you enough flexibility so that you do not have to write your own custom controls. For more information, see Customizing the Appearance of an Existing Control by Creating a ControlTemplate.

## Triggers

A trigger sets properties or starts actions, such as an animation, when a property value changes or when an event is raised. Style, ControlTemplate, and DataTemplate all have a Triggers property that can contain a set of triggers. There are various types of triggers.

### Property Triggers

A Trigger that sets property values or starts actions based on the value of a property is called a property trigger.

To demonstrate how to use property triggers, you can make each ListBoxItem partially transparent unless it is selected. The following style sets the Opacity value of a ListBoxItem to 0.5. When the IsSelected property is true, however, the Opacity is set to 1.0:

<Style TargetType="ListBoxItem">
<Setter Property="Opacity" Value="0.5" />
<Setter Property="MaxHeight" Value="75" />
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Opacity" Value="1.0" />
</Trigger>

  </Style.Triggers>
</Style>


This example uses a Trigger to set a property value, but note that the Trigger class also has the EnterActions and ExitActions properties that enable a trigger to perform actions.

Notice that the MaxHeight property of the ListBoxItem is set to 75. In the following illustration, the third item is the selected item:

### EventTriggers and Storyboards

Another type of trigger is the EventTrigger, which starts a set of actions based on the occurrence of an event. For example, the following EventTrigger objects specify that when the mouse pointer enters the ListBoxItem, the MaxHeight property animates to a value of 90 over a 0.2 second period. When the mouse moves away from the item, the property returns to the original value over a period of 1 second. Note how it is not necessary to specify a To value for the MouseLeave animation. This is because the animation is able to keep track of the original value.

<EventTrigger RoutedEvent="Mouse.MouseEnter">
<EventTrigger.Actions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation
Duration="0:0:0.2"
Storyboard.TargetProperty="MaxHeight"
To="90"  />
</Storyboard>
</BeginStoryboard>
</EventTrigger.Actions>
</EventTrigger>
<EventTrigger RoutedEvent="Mouse.MouseLeave">
<EventTrigger.Actions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation
Duration="0:0:1"
Storyboard.TargetProperty="MaxHeight"  />
</Storyboard>
</BeginStoryboard>
</EventTrigger.Actions>
</EventTrigger>



In the following illustration, the mouse is pointing to the third item:

### MultiTriggers, DataTriggers, and MultiDataTriggers

In addition to Trigger and EventTrigger, there are other types of triggers. MultiTrigger allows you to set property values based on multiple conditions. You use DataTrigger and MultiDataTrigger when the property of your condition is data-bound.

## Shared Resources and Themes

A typical Windows Presentation Foundation (WPF) application might have multiple user interface (UI) resources that are applied throughout the application. Collectively, this set of resources can be considered the theme for the application. Windows Presentation Foundation (WPF) provides support for packaging user interface (UI) resources as a theme by using a resource dictionary that is encapsulated as the ResourceDictionary class.

Windows Presentation Foundation (WPF) themes are defined by using the styling and templating mechanism that Windows Presentation Foundation (WPF) exposes for customizing the visuals of any element.

Windows Presentation Foundation (WPF) theme resources are stored in embedded resource dictionaries. These resource dictionaries must be embedded within a signed assembly, and can either be embedded in the same assembly as the code itself or in a side-by-side assembly. In the case of PresentationFramework.dll, the assembly which contains Windows Presentation Foundation (WPF) controls, theme resources are in a series of side-by-side assemblies.

The theme becomes the last place to look when searching for the style of an element. Typically, the search will begin by walking up the element tree searching for an appropriate resource, then look in the application resource collection and finally query the system. This gives application developers a chance to redefine the style for any object at the tree or application level before reaching the theme.

You can define resource dictionaries as individual files that enable you to reuse a theme across multiple applications. You can also create swappable themes by defining multiple resource dictionaries that provide the same types of resources but with different values. Redefining these styles or other resources at the application level is the recommended approach for skinning an application.

To share a set of resources, including styles and templates, across applications, you can create a XAML file and define a ResourceDictionary. For example, take a look at the following illustration that shows part of the Styling with ControlTemplates Sample:

If you look at the XAML files in the sample, you will notice that the files all have the following:

<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Shared.xaml" />
</ResourceDictionary.MergedDictionaries>


It is the sharing of shared.xaml, which defines a ResourceDictionary that contains a set of style and brush resources that enables the controls in the sample to have a consistent look.