Wpf custom control template - relative font size

I am creating a custom WPF control that let's say for simplicity sake has a vertical stack panel with a "title" TextBlock, followed by a ContentPresenter. I want the font size for the "title" to be 5 Points LARGER than the size used in the content, which is inherited by whatever container the user places this control in.

How can I specify a font size in the control template for the header element using a relative value without exposing a property like "TitleFontSize" to the user? I want do "add 5".

I tried using a ScaleTransform on the header text block with mixed results (the text block scaled fine but the orientation was modified - I had the text right-justified and it moved "off the control" area when scaled). Also, I am not sure if scale transform would be approprite here.

From stackoverflow

• I did it with an IValueConverter as follows:

  Created a class FontSizeConverter that derives from IValueConverter. The Convert method adds 10 to the value, and the ConvertBack method subtracts 10.

  public class FontSizeConverter : IValueConverter
  {
  
   #region IValueConverter Members
  
   public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
   {
    return (double)value + 12.0;
   }
  
   public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
   {
    return (double)value - 12.0;
   }
  
   #endregion
  }
  

  Next, I declaried an instance of this class in the XAML template for the control:

  <Style.Resources>
          <local:FontSizeConverter x:Key="fontSizeConverter"/>
  </Style.Resources>
  

  And Finnaly, the FontSize binding uses this converter applied to the inherited FontSize property:

  <TextBlock FontSize="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=FontSize, Converter={StaticResource fontSizeConverter}}" 
                                     Grid.Row="0" Text="{Binding RelativeSource={RelativeSource TemplatedParent}, 
                                     Path=Date.Day}" HorizontalAlignment="Right" VerticalAlignment="Top" Padding="2" Margin="2" >
   </TextBlock>
  

  This works. But I still do not know if this is the correct answer. Let me know if there is a better way, or if this is appropriate.

  Ian Oakes : Looks good to me, the only change I would make is to pass the value that's being added to the FontSize through the ConverterParameter, rather than hard coding it in the converter.

  Donnelle : As Ian said, I'd add a ConverterParameter but otherwise that looks good.

• A more generic way

  Value converter

  public class MathConverter : IValueConverter
  {
      public object Convert( object value, Type targetType, object parameter, CultureInfo culture )
      {
          return (double)value + double.Parse( parameter.ToString() );
      }
  
      public object ConvertBack( object value, Type targetType, object parameter, CultureInfo culture )
      {
          return null;
      }
  }
  

  Converter Resource

  <my:MathConverter x:Key="MathConverter" />
  

  XAML

  <TextBlock FontSize="{Binding
                       RelativeSource={RelativeSource AncestorType={x:Type Window}},
                       Path=FontSize,
                       Converter={StaticResource MathConverter},
                       ConverterParameter=2}" />