I need to make a Button for bolding selected text of a string in TextBox/RichTextBox, but I don't know how to do it with code behind. 
TextBox1.SelectedText = new FontStyle("Bold")
It is not working.
I need to make a Button for bolding selected text of a string in TextBox/RichTextBox, but I don't know how to do it with code behind. 
TextBox1.SelectedText = new FontStyle("Bold")
It is not working.
Try this code for a rich TextBox:
RichTextBox1.Selection.ApplyPropertyValue(TextElement.FontWeightProperty, FontWeights.Bold);