wpf textbox 实现水印

<TextBox Width="200" VerticalContentAlignment="Center" Background="Transparent" Tag="Enter here...">
    <TextBox.Style>
        <Style TargetType="{x:Type TextBox}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type TextBox}">
                        <Grid>
                            <Border Background="White" BorderBrush="#FF7D8683" BorderThickness="1"/>
                            <ScrollViewer x:Name="PART_ContentHost" Margin="5,0,0,0" VerticalAlignment="Center" />
                            <Label Margin="5,0,0,0" x:Name="WaterMarkLabel" Content="{TemplateBinding Tag}" VerticalAlignment="Center"
                                Visibility="Collapsed" Foreground="Red" FontFamily="Arial" HorizontalContentAlignment="Center" VerticalContentAlignment="Center" />
                        </Grid>
                        <ControlTemplate.Triggers>
                            <MultiTrigger>
                                <MultiTrigger.Conditions>
                                    <Condition Property="Text" Value=""/>
                                </MultiTrigger.Conditions>
                                <Setter Property="Visibility" TargetName="WaterMarkLabel" Value="Visible"/>
                            </MultiTrigger>
                            <MultiTrigger>
                                <MultiTrigger.Conditions>
                                    <Condition Property="IsKeyboardFocused" Value="True"/>
                                </MultiTrigger.Conditions>
                                <Setter Property="Visibility" TargetName="WaterMarkLabel" Value="Collapsed"/>
                            </MultiTrigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </TextBox.Style>
</TextBox>