How is the attached property TextBlock.FontSize used?

Question

I have the following simple code (see below) I copied from a book. But I have a couple of questions about the line <Grid TextBlock.FontSize="48">.

1. From what I gather, TextBlock.FontSize is an attached property but I initially thought that attached properties were meant to reference parent objects (i.e. when the Grid.Row attached property references the parent Grid element). But from how it is used here it may be that my understanding is incorrect? Is this an attached property and if so can it be used for child elements?

2. Second, TextBlock.FontSize is set on the grid. But, no where in the xaml do I use a TextBlock element (that I know of). I only used Buttons with Content defined. Yet if I change the TextBlock.FontSize to a different value the font size changes. Therefore, how is TextBlock.FontSize being used? Where is the TextBlock?

Thank you in advance.

<Window x:Class="UseAGrid.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid TextBlock.FontSize="48">
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="250" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="2*" />
            <ColumnDefinition Width="auto" />
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Button Grid.RowSpan="2"
                Content="2 Rows" />
            <GridSplitter Grid.Row="0"
                          Grid.RowSpan="2"
                          Grid.Column="1"
                          Width="1"
                          Background="Green"
                          ResizeBehavior="PreviousAndNext"
                          ResizeDirection="Columns" />
        <Button Grid.Column="2"
                Grid.ColumnSpan="2"
                Content="2 Columns" />
        <Button Grid.Row="1"
                Grid.Column="2"
                Content="1,2" />
        <Button Grid.Row="1"
                Grid.Column="3"
                Content="1,3" />
    </Grid>
</Window>

Answer 1

TextBlock.FontSize is not an attached property, it's just a regular dependency property. The MSDN documentation is pretty good at listing attached properties for a control and FontSize is not one of them (it doesn't even have any).

It is however an inheritable property. Look at the dependency property information for it and you'll see that it inherits its value. What this allows us to do is set the value of the property in an ancestor and all descendant controls that relies on this property will inherit the same value as long as they don't explicitly set the value to something else.

The controls it applies to doesn't have to be explicitly instantiated by you, it also applies to styles, templates, content presenters, etc. So in your case, the content of your buttons is text so the TextBlocks used to display that text will also inherit the font size.

See Property Value Inheritance for more information.