vb.net error "startindex cannot be larger than length of string" I reversed text direction when typing in a textbox from ltr, to rtl -

i using visual studio 2010 , have been having error saying "startindex cannot larger length of string" reversed text direction when typing in textbox left right.. right left. backspace supposed delete texts on left of blinking cursor, since changed text direction, want backspace delete on left side of blinking cursor instead. example... when entered 4 numbers, 4321 moved cursor in middle of 3 , 2 press backspace 3 times, generates said error.here our code here our error , here our code

  private sub textbox3_keypress(sender object, e system.windows.forms.keypresseventargs) handles textbox3.keypress     if e.keychar = chr(8)         dim tb textbox = ctype(sender, textbox)         dim stext string = tb.text         dim isel_start_pos int64         if stext.length > 0             if tb.selectionstart = 0                 stext = stext.substring(1)                 isel_start_pos = 0             else                 stext = stext.substring(0, tb.selectionstart) & _                         stext.substring(tb.selectionstart + 1)                 isel_start_pos = tb.selectionstart             end if         end if         tb.text = stext         tb.selectionstart = isel_start_pos         e.handled = true     end if end sub private sub textbox3_textchanged(sender system.object, e system.eventargs) handles textbox3.textchanged      if textbox3.selectionstart > 0         textbox3.selectionstart = textbox3.selectionstart - 1     end if end sub