Combination of WPF TextBox and RichTextBox

I am writing a small editor, based on a WPF (c#) TextBox. For some functionality the method getRectFromCharacterIndex(), which is only implemented in the TextBox-class is necessary. Furthermore I've got to do some syntax-highlighting in my progra开发者_高级运维m. Normally this is easy to handle in a TextBlock or a RichTextBox. Unfortunately these two Controls don't include Methods like the one mentioned above. Has anyone an idea, how to format and/or color single Characters in a normal TextBox with a small workaround?

Thank you very much for your answers!

I did the same effort in the past and I decided to move to this: http://wiki.sharpdevelop.net/AvalonEdit.ashx almost all the work you need is done here.