public class NotEmptyStringRule : ValidationRule
    {
        public string FieldName { get; set; }

        public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
        {
            return string.IsNullOrEmpty((string)value) ? new ValidationResult(false, "Значение " + FieldName +
                " должно быть заполнено") : ValidationResult.ValidResult;
        }
    }

            <TextBox Name="txtBoxFirstName" Margin="7,5,7,5" Grid.Column="1">
                <TextBox.Text>
                    <Binding Path="FirstName" NotifyOnValidationError="True" UpdateSourceTrigger="PropertyChanged">
                        <Binding.ValidationRules>
                            <valid:NotEmptyStringRule FieldName="FirstName" />
                        </Binding.ValidationRules>
                    </Binding>
                </TextBox.Text>
            </TextBox>

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            BindingExpression be = this.GetBindingExpression(Window.DataContextProperty);
            //be.UpdateSource();
        }