WPF中PasswordBox控件怎么显示提示信息？

Question

WPF中PasswordBox控件怎么显示提示信息？如显示：请输入密码 
源码这样写的，但抛个异常
 <DockPanel  HorizontalAlignment="Center" Height="23" Width="167" Background="White" Margin="115,92,109,67">
                    <PasswordBox Height="23" HorizontalAlignment="Left" Name="pwdpassword" VerticalAlignment="Top" Width="167">
                    <!--当文本框为空时，显示提示文字-->
                        <PasswordBox.Resources>
                        <VisualBrush x:Key="HelpBrush1"  Opacity="0.3" Stretch="None" AlignmentX="Left">
                            <VisualBrush.Visual>
                                <TextBlock FontStyle="Italic" Text="请输入密码" />
                            </VisualBrush.Visual> 
                        </VisualBrush>
                        </PasswordBox.Resources>
                        <PasswordBox.Style>
                            <Style TargetType="PasswordBox">
                            <Setter Property="Background" Value="White" />
                            <Style.Triggers>
                                <!--<Trigger Property="Password" Value="{x:Null}">
                                        <Setter  Property="Background"  Value="{StaticResource HelpBrush1}"/>
                                </Trigger>-->
                                <!--<Trigger Property="Password" Value="">
                                    <Setter Property="Background" Value="{StaticResource HelpBrush1}" />
                                </Trigger>-->
                            </Style.Triggers>
                        </Style>
                        </PasswordBox.Style>
                    <!--当文本框为空时，显示提示文字-->
                    </PasswordBox>
                </DockPanel>