WPF RichTextBox lock paragraph font

Question

I'm trying to figure out how the documents in RichTextBox work. What I want to achieve is to change selected text font but, not replace a specific one.
So whenever I select text and press a button to replace the font it should keep that one I want but replace all others.

Answer 1

You can call .Selection.GetPropertyValue(TextElement.FontFamilyProperty) to evaluate currently applied font and then apply .Selection.ApplyPropertyValue(TextElement.FontFamilyProperty, "your-font-here") if all necessary conditions are satisfied.