TextDecorations.Underline Property

// Use a linear gradient pen for the underline text decoration. 
private void SetLinearGradientUnderline()
{
    // Create an underline text decoration. Default is underline.
    TextDecoration myUnderline = new TextDecoration();

    // Create a linear gradient pen for the text decoration.
    Pen myPen = new Pen();
    myPen.Brush = new LinearGradientBrush(Colors.Yellow, Colors.Red, new Point(0, 0.5), new Point(1, 0.5));
    myPen.Brush.Opacity = 0.5;
    myPen.Thickness = 1.5;
    myPen.DashStyle = DashStyles.Dash;
    myUnderline.Pen = myPen;
    myUnderline.PenThicknessUnit = TextDecorationUnit.FontRecommended;

    // Set the underline decoration to a TextDecorationCollection and add it to the text block.
    TextDecorationCollection myCollection = new TextDecorationCollection();
    myCollection.Add(myUnderline);
    TextBlock3.TextDecorations = myCollection;
}

<!-- Use a linear gradient pen for the underline text decoration. -->
<TextBlock FontSize="36">the lazy brown dog.
  <TextBlock.TextDecorations>
    <TextDecorationCollection>
      <TextDecoration  
        PenThicknessUnit="FontRecommended">
        <TextDecoration.Pen>
          <Pen Thickness="1.5">
            <Pen.Brush>
              <LinearGradientBrush Opacity="0.5"
                StartPoint="0,0.5"  EndPoint="1,0.5">
                <LinearGradientBrush.GradientStops>
                  <GradientStop Color="Yellow" Offset="0" />
                  <GradientStop Color="Red" Offset="1" />
                </LinearGradientBrush.GradientStops>
              </LinearGradientBrush>
            </Pen.Brush>
            <Pen.DashStyle>
              <DashStyle Dashes="2"/>
            </Pen.DashStyle>
          </Pen>
        </TextDecoration.Pen>
      </TextDecoration>
    </TextDecorationCollection>
  </TextBlock.TextDecorations>
</TextBlock>