For example,

if your fontstyle is bold, then when you set italic, there are many people make mistakes,
and looses bold style while applying bold effect.

Solution:

//store the current font style

FontStyle fs = richTextBox1.Font.Style;

// now you can store all the style whichever you want

fs = fs | FontStyle.Bold|FontStyle.Italic;

//now you can use this fontstyle to apply style of text content.

richTextBox1.Font = new Font("verdana",10.8f, fs);