TextBlock currently only support a limited variations of TextDecorations.

in WPF, custom TextDecoration can be designed by using Pen. This is very useful to create dashed underline.

WPF Example:

<Grid
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Grid.Resources>  
    <Pen x:Key="KeywordDashedUnderlinePen"
         Brush="Black"
         Thickness="2">
      <Pen.DashStyle>
        <DashStyle Dashes="8,4" Offset="0"/>
      </Pen.DashStyle>
      <Pen.StartLineCap>Square</Pen.StartLineCap>
      <Pen.EndLineCap>Square</Pen.EndLineCap>
    </Pen>
    <TextDecoration x:Key="KeywordDashedUnderlineDecoration"
                      Location="Underline"
                      Pen="{StaticResource KeywordDashedUnderlinePen}"
                      PenThicknessUnit="FontRecommended"/>
    <TextDecorationCollection x:Key="KeywordDashedUnderlineCollection">
      <StaticResource ResourceKey="KeywordDashedUnderlineDecoration"/>
    </TextDecorationCollection>
    <Style x:Key="Keyword" TargetType="Run">
      <Setter Property="TextDecorations" Value="{StaticResource KeywordDashedUnderlineCollection}" />
      <Setter Property="FontWeight" Value="SemiBold"/>
      <Setter Property="Foreground" Value="Black" />
    </Style>
  </Grid.Resources>
  <TextBlock Width="400" FontFamily="./#Muli" VerticalAlignment="Center">
    <Run FontFamily="./#Caladea" Foreground="Gray">Text in a TextBlock doesn't have to be a simple string.</Run>
    <LineBreak/>
    <Span FontSize="16">Text can be 
      <Bold>bold</Bold>, 
      <Italic>italic</Italic>,
      <Underline>underlined</Underline>, or
      <Run Style="{StaticResource Keyword}">underlined dashed</Run>.
    </Span>
  </TextBlock>
</Grid>