WPF TextBox MaxLength警告

class MaxLengthValidator : ValidationRule
{
    public MaxLengthValidator()
    {

    }

    public int MaxLength
    {
        get;
        set;
    }


    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
    {
        if (value.ToString().Length <= MaxLength)
        {
            return new ValidationResult(true, null);
        }
        else
        {
            //Here you can also play the sound...
            return new ValidationResult(false, \"too long\");
        }

    }
}

<TextBlock x:Name=\"target\" />
<TextBox  Height=\"23\" Name=\"textBox1\" Width=\"120\">
    <TextBox.Text>
        <Binding Mode=\"OneWayToSource\" ElementName=\"target\" Path=\"Text\" UpdateSourceTrigger=\"PropertyChanged\">
            <Binding.ValidationRules>
                <local:MaxLengthValidator MaxLength=\"10\" />
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>