<Window.Resources>

        <local:L2BConverter x:Key="cvtr"/>

        <Style TargetType="TextBox">

            <Style.Triggers>

                <DataTrigger Binding="{Binding RelativeSource={x:Static RelativeSource.Self},

                                               Path=Text.Length,

                                               Converter={StaticResource cvtr}}"

                             Value="false">

                    <Setter Property="BorderBrush" Value="Red"/>

                    <Setter Property="BorderThickness" Value="1"/>

                </DataTrigger>

            </Style.Triggers>

        </Style>

    </Window.Resources>

    <StackPanel>

        <TextBox Margin="5"/>

        <TextBox Margin="5,0"/>

        <TextBox Margin="5"/>

    </StackPanel>

    public class L2BConverter : IValueConverter

    {

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)

        {

            int textLength = (int)value;

            return textLength >  ? true : false;

        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)

        {

            throw new NotImplementedException();

        }

    }