wpf onpropertychanged not working

                
                        <TextBox x:Name="textBox" HorizontalAlignment="Left" Height="23" Margin="86,44,0,0" TextWrapping="Wrap" Text="{Binding Path=BuyTh, Mode=TwoWay, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Width="120"/>