WPF TextBlock下划线

 <TextBlock Text="H" TextDecorations="Underline"/>

YourTextBlock.TextDecorations = System.Windows.TextDecorations.Underline;

        <TextBlock VerticalAlignment="Bottom" 
                   HorizontalAlignment="Center" 
                   Margin="40" 
                   Height="40" 
                   FontSize="16" 
                   Tapped="TextBlock_Tapped"
                   Text="Text"
                   Foreground="{StaticResource LightBlue}">
            <Underline>
                <Run Text="Text"/>
            </Underline>
        </TextBlock>

<DockPanel LastChildFill="False">
    <TextBlock DockPanel.Dock="Top" x:Name="blockToUnderline" Text="H" Width="76" />
    <Rectangle DockPanel.Dock="Top" Fill="Black" Height=1 Width="{Binding ElementName=blockToUnderline, Path=ActualWidth}" />
</DockPanel>