Templates

Most WPF controls are themselves composed of multiple, simpler, controls. For example, a <Button> is composed of a <Border> and whatever content you place inside the button. A simplified version of this structure appears below (I removed the styling information and the VisualState components responsible for presenting the button differently when it is enabled, disabled, hovered on, or clicked):

<Border TextBlock.Foreground="{TemplateBinding Foreground}"
        x:Name="Border"
        CornerRadius="2"
        BorderThickness="1">
  <Border.BorderBrush>
    <LinearGradientBrush StartPoint="0,0"
                          EndPoint="0,1">
      <LinearGradientBrush.GradientStops>
        <GradientStopCollection>
          <GradientStop Color="{DynamicResource BorderLightColor}"
                        Offset="0.0" />
          <GradientStop Color="{DynamicResource BorderDarkColor}"
                        Offset="1.0" />
        </GradientStopCollection>
      </LinearGradientBrush.GradientStops>
    </LinearGradientBrush>
  </Border.BorderBrush>
  <Border.Background>
    <LinearGradientBrush EndPoint="0.5,1"
                          StartPoint="0.5,0">
      <GradientStop Color="{DynamicResource ControlLightColor}"
                    Offset="0" />
      <GradientStop Color="{DynamicResource ControlMediumColor}"
                    Offset="1" />
    </LinearGradientBrush>
  </Border.Background>
  <ContentPresenter Margin="2"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Center"
                    RecognizesAccessKey="True" />
</Border>

This has some implications for working with the control - for example, if you wanted to add rounded corners to the <Button>, they would actually need to be added to the <Border> inside the button. This can be done by nesting styles, i.e.:

<Grid>
    <Grid.Resources>
        <Style TargetType="Button">
            <Style.Resources>
                <Style TargetType="Border">
                    <Setter Property="CornerRadius" Value="25"/>
                </Style>
            </Style.Resources>
        </Style>
    </Grid.Resources>
    <Button>I have rounded corners now!</Button>
</Grid>

Note how the <Style> targeting the <Border> is nested inside the Resources of the <Style> targeting the <Button>? This means that the style rules for the <Border> will only be applied to <Border> elements that are part of a <Button>.

Templates

Above I listed a simplified version of the XAML used to create a button. The full listing can be found in the Microsoft Documentation

What’s more, you can replace this standard rendering in your controls by replacing the Template property. For example, we could replace our button with a super-simple rounded <Border> that nested a <TextBlock> that does word-wrapping of the button content:

<Button>
    <Button.Template>
        <ControlTemplate>
            <Border CornerRadius="25">
                <TextBlock TextWrapping="Wrap">  
                    <ContentPresenter Margin="2"
                        HorizontalAlignment="Center"
                        VerticalAlignment="Center"
                        RecognizesAccessKey="True" />
                </TextBlock>
            </Border>
        </ControlTemplate>
    </Button.ControlTemplate>
    This is a simple button!
</Button>

The <ContentPresenter> is what presents the content nested inside the button - in this case, the text This is a simple button!. Of course, this super-simple button will not change its appearance when you hover over it or click it, or when it is disabled. But it helps convey the idea of a <ControlTemplate>. As with any other property, you can also set the Template property of a control using a <Setter> within a <Style> targeting that element.

If you only need a simple tweak - like applying word-wrapping to the text of a button, it often makes more sense to supply as content a control that will do so, i.e.:

<Button>
    <TextBlock TextWrapping="Wrap">
        I also wrap text!
    </TextBlock>
</Button>

This allows the <Button> to continue to use the default ControlTemplate while providing the desired word-wrapping with a minimum of extra code.

A similar idea appears with <DataTemplate>, which allows you to customize how bound data is displayed in a control. For example, we often want to display the items in a <ListBox> in a different way than the default (a <TextBlock> with minimal styling). We’ll visit this in the upcoming [binding lists]({{ref “2-desktop-development/04-data-binding/04-binding-lists”}}) section.