WPF中给TextBox/TextBlock设置提示文本

<TextBox.Resources>
    <VisualBrush x:Key="HintText" TileMode="None" Opacity="0.5" Stretch="None" AlignmentX="Left">
        <VisualBrush.Visual>
            <TextBlock FontStyle="Italic" FontSize="11" Text="双击进入选择"/>
        </VisualBrush.Visual>
    </VisualBrush>
</TextBox.Resources>
<TextBox.Style>
    <Style TargetType="TextBox">
        <Style.Triggers>
            <Trigger Property="Text" Value="{x:Null}">
                <Setter Property="Background" Value="{StaticResource HintText}"/>
            </Trigger>
            <Trigger Property="Text" Value="">
                <Setter Property="Background" Value="{StaticResource HintText}"/>
            </Trigger>
        </Style.Triggers>
    </Style>
</TextBox.Style>