WPF richTextBox question

The trick I found uses the TextPointer class and its GetCharacterRec method.

RichTextBox holds a FlowDocument. Text in flow documents is contained in a Run object (bit of a simplification, but it works). The code finds the TextPointer at the start of the first Run. It then gets the bounding rectangle of that first character. Next the code walks forward one character at a time, gets a new bounding rectangle and checks if the bottom of the new rectangle is different from the original rectangle. If the bottom is different then we are on a new line. TextPointer can then get the text either before or after the break.

public partial class Window1 : Window{    public Window1()    {        InitializeComponent();    }    private void inspect(object sender, RoutedEventArgs e)    {        TextPointer pointer = FindRun(inBox.Document);        string textAfterBreak = FindBreak(pointer);        outBox.Text = textAfterBreak;    }    private string FindBreak(TextPointer pointer)    {        Rect rectAtStart = pointer.GetCharacterRect(LogicalDirection.Forward);        pointer = pointer.GetNextInsertionPosition(LogicalDirection.Forward);        Rect currentRect = pointer.GetCharacterRect(LogicalDirection.Forward);        while (currentRect.Bottom == rectAtStart.Bottom)        {            pointer = pointer.GetNextInsertionPosition(LogicalDirection.Forward);            currentRect = pointer.GetCharacterRect(LogicalDirection.Forward);        }        string textBeforeBreak = pointer.GetTextInRun(LogicalDirection.Backward);        string textAfterBreak = pointer.GetTextInRun(LogicalDirection.Forward);        return textAfterBreak;    }    private TextPointer FindRun(FlowDocument document)    {        TextPointer position = document.ContentStart;        while (position != null)        {            if (position.Parent is Run)                break;            position = position.GetNextContextPosition(LogicalDirection.Forward);        }        return position;    }}<Window x:Class="LineBreaker.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>        <Grid.RowDefinitions>            <RowDefinition></RowDefinition>            <RowDefinition></RowDefinition>            <RowDefinition></RowDefinition>        </Grid.RowDefinitions>        <RichTextBox Grid.Row="0" Name="inBox">        </RichTextBox>        <Button Grid.Row="1" Click="inspect">Find Break</Button>        <TextBox Name="outBox" Grid.Row="2"/>    </Grid></Window>

http://msdn.microsoft.com/en-us/library/system.windows.documents.textpointer.getlinestartposition.aspx

TextPointer startOfFirstLine = richTextBox.Document.ContentStart;TextPointer startOfNextLine = startOfFirstLine.GetLineStartPosition(1);if(startOfNextLine != null){     // At this point what you do with the TextPointer depends on what you define as the position of text.     // If you want to find out how many characters are on the first line ...    int firstLineCharacterCount = new TextRange(startOfFirstLine, startOfNextLine).Text.Length;}

But startOfFirstLine.GetLineStartPosition(1) returns null