** GotFocus
thisform.old_value = this.Value

** In Valid Event
LOCAL ret_val, m_rec
ret_val = .t.
IF LastKey() # 27 && if the user press Esc
   IF thisform.old_value # this.Value
      SELECT MyTable
      old_record = RecNo()
      SEEK this.Value  && To Move the pointer
      GOTO old_record
   ENDIF  
ENDIF
RETURN ret_val

*** In Error event of the TextBox
  LPARAMETERS nError, cMethod, nLine
  IF nError = 1884 && Uniqueness is violated
     MessageBox("Record with this value already exiists",0,"Warning")
     this.Value = thisform.old_value
  ENDIF