How to read a WPF RichTextBox Content?

The following code creates a RichTextBox in WPF and adds a flow document to it. Code also adds a button in XAML.

<Window x:Class="RichTextBoxSample.Window1"
    xmlns=http://schemas.microsoft.com/winfx/2006/xaml/presentation
    xmlns:x=http://schemas.microsoft.com/winfx/2006/xaml
    Title="Window1" Height="300" Width="300">
    <Grid>
            <FlowDocument>
                <Paragraph>
                    <Run>First paragraph goes here. Type some text here.</Run>
                </Paragraph>
                <Paragraph>
                    <Run>Second paragraph goes here. Type some text here.</Run>
                </Paragraph>          
            </FlowDocument>
        </RichTextBox>
        <Button Height="36" Margin="12,0,0,34" Name="button1" VerticalAlignment="Bottom"
                HorizontalAlignment="Left" Width="97" Click="button1_Click">Get Contents</Button>
    </Grid>
</Window>

On button click event handler, which looks like following, we are going to read contents of a RichTextBox. As you can see from the code below, we use RichTextBox.ContentStart and RichTextBox.ContentEnd to get the entire content and later display in a MessageBox.

private void button1_Click(object sender, RoutedEventArgs e)
{
    TextRange textRange = new TextRange(RTB.Document.ContentStart,
        RTB.Document.ContentEnd  );
    MessageBox.Show(textRange.Text);