RichTextBox.Selection Property

// Create a simple FlowDocument to serve as the content input for the construtor.
FlowDocument flowDoc = new FlowDocument(new Paragraph(new Run("Simple FlowDocument")));
// After this constructor is called, the new RichTextBox rtb will contain flowDoc.
RichTextBox rtb = new RichTextBox(flowDoc);
// This call will select the entire contents of the RichTextBox.
rtb.SelectAll();
// This call returns the current selection (which happens to be the entire contents
// of the RichTextBox) as a TextSelection object.
TextSelection currentSelection = rtb.Selection;