How do get the current line text to cursor from a Silverlight 4 RichTextBox Control - silverlight-4.0

Question

In the Winforms RichTextBox control I have previously used the GetLineFromCharIndex method and the GetFirstCharIndexOfCurrentLine to work out the start and end points of the typed text on he current line.
I am struggling with the new RichTextBox control in Silverlight 4 as there doesn't appear to be equivalent methods. GetPositionFromPoint is available but seems a but clunky.
Cheers.
Updated...I have gone someway to making this work but this requires me to use the Select method of the control, this feels very wrong...
private string GetCurrentLine()
{
TextPointer prevSelStart = richTextBox1.Selection.Start;
Point lineStart = new Point(0, prevSelStart.GetCharacterRect(LogicalDirection.Forward).Y);
TextPointer prevSelEnd = richTextBox1.Selection.End;
TextPointer currentLineStart = richTextBox1.GetPositionFromPoint(lineStart);
//need to find a way to get the text between two textpointers
//other than performing a temporary selection in the rtb
richTextBox1.Selection.Select(currentLineStart, prevSelStart);
string text = richTextBox1.Selection.Text;
//revert back to previous selection
richTextBox1.Selection.Select(prevSelStart, prevSelEnd);
return text;
}

Answer 1

I don't think you can't avoid the selection, it's a proper way to do it (the "selection" is just a logical one), but you can avoid GetPositionFromPoint with TextPointer.GetNextInsertionPosition(LogicalDirection ): Start from richTextBox1.Selection.Start and move towards the beginning of the line (char != '\n')

Answer 2

I've needed to figure out when I was on the top line or bottom line of the RTB. To do this I used the GetCharacterRect methods then compared the tops to see if it was on the last line or first line.
You could do the same and use a text pointer to move through the text and number of times the tops don't match.
Here's code to see if the cursor is on the first or last lines:
private bool IsCursorOnFirstLine()
{
TextPointer contentStart = this.ContentStart;
TextPointer selection = this.Selection.End;
Rect startRect = contentStart.GetCharacterRect(LogicalDirection.Forward);
Rect endRect = selection.GetCharacterRect(LogicalDirection.Forward);
return startRect.Top == endRect.Top;
}
private bool IsCursorOnLastLine()
{
TextPointer start = this.Selection.Start;
TextPointer end = this.ContentEnd;
Rect startRect = start.GetCharacterRect(LogicalDirection.Forward);
Rect endRect = end.GetCharacterRect(LogicalDirection.Backward);
return startRect.Top == endRect.Top;
}