Set a property of a nested element in an WPF style

Solution 1:

I know 3 options for customizing template in the way you are looking for

1) create a userControl derived from ToggleButton, add a RadiusValue dependency property there (of type CornerRadius) and use it in control template: CornerRadius="{TemplateBinding RadiusValue}".

2) use a dynamic resource.

after running into a roadblock with dynamic resources (Wpf dynamic resource lookup for Validation.ErrorTemplate) I prefer the 3rd one

3) use an attached dependency property

at first I created an attached DP of type CornerRadius (with default radius = 3)

public static class Attached
{
    public static readonly DependencyProperty RadiusValueProperty =
        DependencyProperty.RegisterAttached("RadiusValue", typeof (CornerRadius), typeof (Attached), new FrameworkPropertyMetadata(new CornerRadius(3)));


    public static void SetRadiusValue(DependencyObject element, CornerRadius value)
    {
        element.SetValue(RadiusValueProperty, value);
    }

    public static CornerRadius GetRadiusValue(DependencyObject element)
    {
        return (CornerRadius)element.GetValue(RadiusValueProperty);
    }
}

after that I modified custom ToggleButton template:

<Style x:Key="ToggleRadioButton" 
        TargetType="{x:Type ToggleButton}" 
        BasedOn="{StaticResource {x:Type ToggleButton}}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ToggleButton">
                <Border BorderBrush="Blue" 
                Background="{TemplateBinding Background}"
                Padding="15,0" 
                BorderThickness="1" 
                CornerRadius="{Binding Path=(local:Attached.RadiusValue), 
                    RelativeSource={RelativeSource TemplatedParent}}"> 
                    <ContentPresenter HorizontalAlignment="Center"
                                VerticalAlignment="Center"/>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <Trigger Property="IsChecked" Value="True">
            <Setter Property="Background" Value="Blue"/>
            <Setter Property="Foreground" Value="White"/>
        </Trigger>
        <Trigger Property="IsChecked" Value="False">
            <Setter Property="Background" Value="White"/>
            <Setter Property="Foreground" Value="Black"/>
        </Trigger>
    </Style.Triggers>
</Style>

the only change (except key) is

CornerRadius="{Binding Path=(local:Attached.RadiusValue), 
               RelativeSource={RelativeSource TemplatedParent}}">

and finally derived style for Left RadioButton, based on ToggleRadioButton

<Style x:Key="ToggleRadioButtonLeft" 
        TargetType="{x:Type ToggleButton}" 
        BasedOn="{StaticResource ToggleRadioButton}">
    <Setter Property="local:Attached.RadiusValue" Value="10,0,0,10"/>
</Style>