how to change the Selected font by letter by letter in vb.net

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VB

font

how to do this for a richtextbox.
i need the code for my text editor

This code is a bit buggy. \/ \/ \/ \/ \/ \/ \/

Dim fontStyle As System.Drawing.FontStyle = RichTextBox1.SelectionFont.Style
          Dim fontName As String = ComboBox1.Text
          Dim fontSize As Integer = ComboBox2.Text
          Dim FS As New Font(fontName, fontSize, fontStyle)
          RichTextBox1.SelectionFont = FS

this code also if the fist letter of it is Bold and the other part is underline it will make it bold and I am just tiring to change the size of the Selected font.
can you please help me, Please :).