With RichTextBox1
    Dim firstindex As Long = _
       .GetCharIndexFromPosition(.ClientRectangle.Location)
 
    Dim posend As Point = .ClientRectangle.Location
    posend.Offset(New Point(.ClientRectangle.Width, .ClientRectangle.Height))
 
    Dim lastindex As Long = .GetCharIndexFromPosition(posend)
End With