<!--A RichTextBox with intial content in it.-->
<RichTextBox VerticalScrollBarVisibility="Auto">
    <Paragraph>
        A RichTextBox with <Bold>initial content</Bold> in it.
    </Paragraph>
</RichTextBox>

'A RichTextBox with intial content in it.
Private Sub ContentRTB()
    'Create a new RichTextBox with its VerticalScrollBarVisibility property set to Auto.
    Dim MyRTB As New RichTextBox()
    MyRTB.VerticalScrollBarVisibility = ScrollBarVisibility.Auto

    ' Create a Run of plain text and some bold text.
    Dim myRun1 As New Run()
    myRun1.Text = "A RichTextBox with "
    Dim myBold As New Bold()
    myBold.Inlines.Add("initial content ")
    Dim myRun2 As New Run()
    myRun2.Text = "in it."

    ' Create a paragraph and add the Run and Bold to it.
    Dim myParagraph As New Paragraph()
    myParagraph.Inlines.Add(myRun1)
    myParagraph.Inlines.Add(myBold)
    myParagraph.Inlines.Add(myRun2)

    ' Add the paragraph to the RichTextBox.
    MyRTB.Blocks.Add(myParagraph)

    'Add the RichTextBox to the StackPanel.
    MySP.Children.Add(MyRTB)
End Sub

//A RichTextBox with intial content in it.
private void ContentRTB()
{
    //Create a new RichTextBox with its VerticalScrollBarVisibility property set to Auto.
    RichTextBox MyRTB = new RichTextBox();
    MyRTB.VerticalScrollBarVisibility = ScrollBarVisibility.Auto;

    // Create a Run of plain text and some bold text.
    Run myRun1 = new Run();
    myRun1.Text = "A RichTextBox with ";
    Bold myBold = new Bold();
    myBold.Inlines.Add("initial content ");
    Run myRun2 = new Run();
    myRun2.Text = "in it.";

    // Create a paragraph and add the Run and Bold to it.
    Paragraph myParagraph = new Paragraph();
    myParagraph.Inlines.Add(myRun1);
    myParagraph.Inlines.Add(myBold);
    myParagraph.Inlines.Add(myRun2);

    // Add the paragraph to the RichTextBox.
    MyRTB.Blocks.Add(myParagraph);

    //Add the RichTextBox to the StackPanel.
    MySP.Children.Add(MyRTB);
}