' Use a linear gradient pen for the underline text decoration.
Private Sub SetLinearGradientUnderline()
    ' Create an underline text decoration. Default is underline.
    Dim myUnderline As New TextDecoration()

    ' Create a linear gradient pen for the text decoration.
    Dim myPen As 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.
    Dim myCollection As New TextDecorationCollection()
    myCollection.Add(myUnderline)
    TextBlock3.TextDecorations = myCollection
End Sub