WPF：在textBox控件中添加提示词

<TextBox x:Name="userName" Width="250" Height="40" Grid.Row="1" VerticalContentAlignment="Center" HorizontalContentAlignment="Center" Grid.ColumnSpan="2" FontSize="20" Foreground="Indigo" TabIndex="0" BorderThickness="1">
<TextBox.Resources>
<VisualBrush x:Key="HelpBrush" TileMode="None" Opacity="0.3" Stretch="None" AlignmentX="Left">
<VisualBrush.Visual>
<TextBlock FontStyle="Italic" Text="请输入用户名"/>
</VisualBrush.Visual>
</VisualBrush>
</TextBox.Resources>
<TextBox.Style>
<Style TargetType="TextBox">
<Style.Triggers>
<Trigger Property="Text" Value="{x:Null}">
<Setter Property="Background" Value="{StaticResource HelpBrush}"/>
</Trigger>
<Trigger Property="Text" Value="">
<Setter Property="Background" Value="{StaticResource HelpBrush}"/>
</Trigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>