Wpf richtextbox prevent undo

Hello!

I have a RichTextBox control. I use it for syntax highlighting in my text.

The problem is: after a syntax is highlighted, when user press Ctrl+Z (calls Undo action)... control "unhighlights" my text.

For highlighting I use Select method and SelectionColor property. How to suppress writting actions into the undo stack of a RichTextBox during SelectionColor changes? Is it possible?work.

I tried using the LockCurrentUndoUnit method on the highlight logic to solve this problem, but it did not work.