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How to: Replace the Default Content Host for a RichTextBox

This example shows how to use Windows Presentation Foundation (WPF) styles to replace the default content host for a RichTextBox.

The content host is the element that renders the contents of the RichTextBox. The default control template for a RichTextBox specifies a ScrollViewer as the content host.

In cases where the scrolling features provided by a ScrollViewer are unwanted or unneeded, a lighter-weight AdornerDecorator element may be specified as the content host for a RichTextBox. ScrollViewer and AdornerDecorator are the only supported elements for the content host.

For a working sample that demonstrates this example, see Replace the Default Content Host for a RichTextBox Sample.

Example

The ControlTemplate for a RichTextBox must contain exactly one element that is tagged as the content host element. To tag an element as the content host, assign it the special name PART_ContentHost. The content host element must be either a ScrollViewer or an AdornerDecorator. The content host element may not host any child elements.

The following Extensible Application Markup Language (XAML) example defines a style that overrides the default control template for a RichTextBox. This style is compatible with elements that descend from TextBoxBase. In the example, an AdornerDecorator is specified as the content host.

<Window.Resources>
  <Style x:Key="TextBoxNoScrollViewer" TargetType="{x:Type TextBoxBase}">
    <Setter Property="Template">
      <Setter.Value>
        <ControlTemplate TargetType="{x:Type TextBoxBase}">
          <Border 
            CornerRadius="2" 
            Background="{TemplateBinding Background}" 
            BorderThickness="{TemplateBinding BorderThickness}" 
            BorderBrush="{TemplateBinding BorderBrush}"  
          >
            <!-- 
            The control template for a TextBox or RichTextBox must
            include an element tagged as the content host.  An element is 
            tagged as the content host element when it has the special name
            PART_ContentHost.  The content host element must be a ScrollViewer,
            or an element that derives from Decorator.  
            -->
            <AdornerDecorator 
              x:Name="PART_ContentHost"
              Focusable="False" 
            />
          </Border>
        </ControlTemplate>
      </Setter.Value>
    </Setter>
  </Style>
</Window.Resources>

The following XAML example defines a RichTextBox that makes use of the previously declared style by using the Style attribute coupled with a static resource reference to the style's x:Key Attribute.

<RichTextBox
  Grid.Column="0"
 
  VerticalScrollBarVisibility="Auto"
  HorizontalScrollBarVisibility="Auto"
  
  Style="{StaticResource TextBoxNoScrollViewer}"      
>
  <FlowDocument>
    <Paragraph>
      RichTextBox styled not to use a ScrollViewer as the content host.
    </Paragraph>
  </FlowDocument>
</RichTextBox>

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