How to set the FontSize property of a TextBlock in a ViewModel

Question

We have made a WPF UserControl with a Label and a nested TextBlock (for textwrapping). Because we need to copy the UserControl after Drag-and-Drop we can't use names in the UserControl-XAML. So we needed to remove all Names from the XAML and use a ViewModel. But with some DependencyProperties of the nested TextBlock we have problems. When we try to set the FonzSize-property per example, it doesn't react. Meanwhile we have found out, that it works, if we define a property with a new Name like "ControlFontSize". But if it's possible we want to use the propertyname "FontSize". Is there a way to handle this?

This is the UserControl-XAML:

<UserControl x:Class="WpfDesignerControlLibrary.ControlLabel"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:WpfDesignerControlLibrary"
             mc:Ignorable="d"
             d:DesignHeight="30"
             d:DesignWidth="150">
    <UserControl.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Resources.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </UserControl.Resources>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Border Style="{StaticResource usercontrolBorderStyle}"
                BorderBrush="{Binding Path=BorderBrush, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                BorderThickness="{Binding Path=BorderThickness, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}">
            <Label Margin="2"
                   HorizontalContentAlignment="{Binding Path=ControlHorizontalContentAlignment, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                   VerticalContentAlignment="{Binding Path=ControlVerticalContentAlignment, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}">
                <TextBlock Text="{Binding Path=Text, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                           FontSize="{Binding Path=ControlFontSize, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                           TextAlignment="{Binding Path=ControlTextAlignment, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                           FontWeight="{Binding Path=FontWeight, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                           FontStyle="{Binding Path=FontStyle, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                           TextDecorations="{Binding Path=TextDecorations, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                           TextWrapping="Wrap"/>
            </Label>
        </Border>
    </Grid>
</UserControl>

That's the property in the CS-File of the UserControl, which gets and sets the ViewModel-property:

public new double FontSize
{
    get { return viewmodel.FontSize; }
    set { viewmodel.FontSize = value; }
}

And this is the ViewModel-Property itself:

public double FontSize
{
    get { return fontSize; }
    set
    {
        fontSize = value;
        RaisePropertyChanged("FontSize");
    }
}

Answer 1

We have found it out ourselves. You can bind the FontSize-property of the TextBlock to it's ancestor - in this case the UserControl itself.

In XAML of the UserControl:

        <TextBlock Text="{Binding Path=Text, Mode=TwoWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
                   FontSize="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type UserControl}}, Path=FontSize}"

And in a testwindow You can then use the FontSize-property of the UserControl as usual. No need for a property with a new Name or DependencyProperty.