Clear any formatting applied to the text with TextRange.ClearAllProperties();

   


<Window x:Class="WpfApplication1.Window1"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  Title="Window1" Height="600" Width="800">
  <Grid>
    <RichTextBox x:Name="rtbTextContent" TextChanged="RichTextBox_TextChanged" />
  </Grid>
</Window>
//File:Window.xaml.vb
Imports System.Windows
Imports System.Windows.Controls
Imports System.Windows.Documents
Imports System.Windows.Media

Namespace WpfApplication1

  Public Partial Class Window1
    Inherits Window
    Public Sub New()
      InitializeComponent()
    End Sub

    Private Sub RichTextBox_TextChanged(sender As Object, e As TextChangedEventArgs)

      Dim textRange As New TextRange(rtbTextContent.Document.ContentStart, rtbTextContent.Document.ContentEnd)
      RemoveHandler rtbTextContent.TextChanged, AddressOf RichTextBox_TextChanged
      textRange.ClearAllProperties()
      ApplyFormatting()
      AddHandler rtbTextContent.TextChanged, AddressOf RichTextBox_TextChanged
    End Sub

    Private Sub ApplyFormatting()
      Dim tp As TextPointer = rtbTextContent.Document.ContentStart
      tp = FindNextString(tp)

      Dim textRangeEnd As TextPointer = tp.GetPositionAtOffset(1, LogicalDirection.Forward)

      Dim tokenTextRange As New TextRange(tp, tp.GetPositionAtOffset(1, LogicalDirection.Forward))

      tokenTextRange.ApplyPropertyValue(TextElement.ForegroundProperty, Brushes.Blue)
    End Sub

    Private Function FindNextString(tp As TextPointer) As TextPointer
      Dim buffer As Char() = New Char(0) {}
      tp.GetTextInRun(LogicalDirection.Forward, buffer, 0, 1)
      Return tp
    End Function
  End Class
End Namespace

1.	Programmatically Insert Text into a RichTextBox
2.	Get selected text from RichTextBox by using RichTextBox.Selection.Text
3.	Get Caret Position in a RichTextBox by using RichTextBox.CaretPosition.GetPositionAtOffset
4.	Get the content from the rich text box.
5.	Add / Remove TextChanged event for RichTextBox
6.	Apply Syntax Highlighting in a Text Control
7.	Select all text from RichTextBox and cut
8.	Use Application Command to edit RichTextBox
9.	Load or Save the Content of a RichTextBox
10.	Display a Password Entry Box and get the input

Clear any formatting applied to the text with TextRange.ClearAllProperties(); : RichTextBox « Windows Presentation Foundation « VB.Net

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