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>
      <LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
        <LinearGradientBrush.GradientStops>
          <GradientStop Offset="0.0" Color="#90DDDD" />
          <GradientStop Offset="1.0" Color="#5BFFFF" />
        </LinearGradientBrush.GradientStops>
      </LinearGradientBrush>
    </Setter.Value>
  </Setter>
</Style>


...


</Window.Resources>

Notice that the above 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 we have overridden 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.

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, 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 instance, 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.

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 preceding 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. If, for instance, 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 on scoping rules for resources, see Resources Overview.

In addition, you can find more information about styles and resources in Shared Resources and Themes later in this overview.

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, so 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"]);

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.

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, we've only discussed the use of setters to set property value. You can also specify event handlers in a style. For more information, see EventSetter.

First, it is important to point out that it is not necessary to use the ControlTemplate to make the ListBox horizontal. A ListBox has an ItemsPanel property that allows you to set an ItemsPanelTemplate, the template that controls the layout of the items of the ListBox. One option is to simply create a ListBox style and set the ItemsPanel property, as in the following example:

<Style TargetType="ListBox">
  <Setter Property="ItemsPanel">
    <Setter.Value>
      <ItemsPanelTemplate>
        <StackPanel Orientation="Horizontal"
                    VerticalAlignment="Center"
                    HorizontalAlignment="Center"/>
      </ItemsPanelTemplate>
    </Setter.Value>
  </Setter>
</Style>

This works just fine and gives us a horizontal ListBox. This example shows that depending on your scenario, there may be options other than to replace the ControlTemplate. For this example, if we want a horizontal ListBox that has additional properties, such as rounded corners, then we need to work with the ControlTemplate of the ListBox.

Before we provide an example to show how to do that, it is important to first explain the concept of a ControlTemplate.

For most controls, there is appearance and behavior. Consider a button: appearance is the raised area that you can press, and the behavior is the Click event that gets raised in response to a click.

Sometimes, there may be a control that provides the behavior that you need but not the appearance that you need. So far, we have shown that you can use style setters to set property values to affect the look of control. However, to change the structure of a control or to set property values on the components that comprise a control, you need to use a ControlTemplate.

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 don't have to write your own custom controls. If you do not define your own ControlTemplate for your control, you get the default template that matches the system theme, which is what gives the Button control its default look.

One thing to keep in mind is that once you create a ControlTemplate for you control, you are replacing the entire ControlTemplate. For example, you may define your Button ControlTemplate the following way.

Note that the ContentPresenter element simply marks where the Content of the Button should go. We will discuss the different pieces in a later section.

<Style TargetType="Button">
  <!--Set to true to not get any properties from the themes.-->
  <Setter Property="OverridesDefaultStyle" Value="True"/>
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="Button">
        <Grid>
          <Ellipse Fill="{TemplateBinding Background}"/>
          <ContentPresenter HorizontalAlignment="Center"
                            VerticalAlignment="Center"/>
        </Grid>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>

When this gets applied, the Button appears as an Ellipse:

Remember that what the Button looks like when it is in focus or pressed is all part of the default appearance of the button that you are replacing. Therefore, depending on your needs, you may want to put in your definition what your button should look like when it is pressed. For a complete example, see Button ControlTemplate Example.

If you are creating a ControlTemplate, the best way to get started is to use the ControlTemplate Examples. If you really need to look at the parts that a control is composed of, you can take a look at the themes file located in Themes or you can use the Show Visual Tree functionality of XAMLPad, an application that is installed with the Windows Software Development Kit (SDK).

Now let's continue with our example and create a ControlTemplate that defines a ListBox that is horizontal and has rounded corners. To replace the ControlTemplate of a control, set the Template property to the new ControlTemplate.

<Style TargetType="ListBox">
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="ListBox">
        <Border CornerRadius="5" Background="{TemplateBinding ListBox.Background}">
          <ScrollViewer HorizontalScrollBarVisibility="Auto">
            <StackPanel Orientation="Horizontal"
                       VerticalAlignment="Center"
                       HorizontalAlignment="Center"
                       IsItemsHost="True"/>
          </ScrollViewer>
        </Border>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>

When you set the Template property this way, it is really no different than setting other control properties using a Style: you are using a Style as a tool to help you set the Template property. This means that another way to set a ControlTemplate is to set the Template property on your control directly. If you do it that way, you would create a ControlTemplate in the Resources section, provide it with an x:Key, and then use it as a static resource. For an example, see the Template property.

As you can see in the above example, the ControlTemplate class has a TargetType property that is similar to the TargetType property of the Style class. However, note that unlike Style and DataTemplate, ControlTemplate objects do not have the notion of an implicit key. In other words, if you have a standalone ControlTemplate with the TargetType property set to a type, the ControlTemplate does not get applied to that type automatically. Also note that the TargetType property is required on a ControlTemplate if the template definition contains a ContentPresenter.

Try experimenting with the ControlTemplate. For example, replace the StackPanel with a WrapPanel, set the HorizontalScrollBarVisibility property of the ScrollViewer to Disabled, and then set the Width of the ListBox to 300. (The WrapPanel only puts items to the next row when the first row runs out of space. If you don't set the HorizontalScrollBarVisibility property of the ScrollViewer to Disabled the first row does not run out of space because you can scroll to the end. As a result, the WrapPanel does not wrap the items.)

In this example, one important property that must be there is the IsItemsHost property. The IsItemsHost property is used to indicate in the template of an ItemsControl (controls such as ListBox that work with a list of items,) where generated elements should go. Setting the property to true on the StackPanel means that any items added to the ListBox go into the StackPanel. Note that this property only works on Panel types.

However, note that when you specify a panel in the ControlTemplate and mark it as the IsItemsHost this way, the ItemsPanel cannot be replaced by the user of the control without using a ControlTemplate. Therefore, only do it that way if you know you wouldn't want the panel to be replaced without the use of a template. Alternatively, you can use the ItemsPresenter element to mark where the items should go and then specify an ItemsPanelTemplate by setting the ItemsPanel property. The ItemsPanelTemplate page has an example that shows you how to do that. For another example that uses the ItemsPresenter, see TreeView ControlTemplate Example.

If you are creating a template for a ContentControl such as a Button, the corresponding element is the ContentPresenter. Similarly, you place this element in the ControlTemplate of your ContentControl type to indicate where the content should be displayed, as demonstrated in the example in the What Is a ControlTemplate? section. For other examples, see Label ControlTemplate Example and ListBoxItem ControlTemplate Example.

Another important thing to notice in the previous example is the Background value that is set to {TemplateBinding ListBox.Background}. This is simply indicating that the Background of the Border should be synchronized with the Background value that is set on the ListBox. A TemplateBinding is similar to a Binding. In fact, a TemplateBinding is more efficient but less functional than a Binding; using a TemplateBinding is equivalent to using a Binding with the Source property set to RelativeSource.TemplatedParent.

You use TemplateBinding in your ControlTemplate when you want to give the user of your control the control over the values of certain properties. TemplateBinding is a markup extension that is represented by TemplateBindingExtension class.

You may have noticed that DataTemplate and ControlTemplate are similar in that their content becomes the visualization of an object. With the ListBox ControlTemplate definition, our application now looks like the following:

To demonstrate how to use triggers to set properties, let's 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 we have also set the MaxHeight property of the ListBoxItem to 75. In the following screenshot, the third item is the selected item:

We just showed that a Trigger sets property values or starts actions based on the value of a property. 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>

For more information, see the Storyboards Overview.

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

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.

For the complete sample discussed in this overview, see Introduction to Styling and Templating Sample.