WPF - Underline some text in Richtextbox

Question

I have a WPF RichTextBox (rtbControl) and the content as following:

This is not an underlined line
This is an underlined line

I want to underline the 2nd line, how can I do it ?

Answer 1

Since RTB's content is a FlowDocument, then you have to create appropriate document structure:

    <RichTextBox>
        <FlowDocument>
            <Paragraph>
                <Run Text="This is not an underline line"/>
            </Paragraph>
            <Paragraph>
                <Underline>
                    <Run Text="This is an underline line"/>
                </Underline>
            </Paragraph>
        </FlowDocument>
    </RichTextBox>

Answer 2

<RichTextBox>
    <FlowDocument>
        <Paragraph>
            This is not an underline line
        </Paragraph>     

        <Paragraph>
            <Underline>This is an underline line</Underline>
        </Paragraph>     

    </FlowDocument>       

</RichTextBox>