WPF 带有提示文本的透明文本框

<TextBox Text="{Binding SearchInfo, UpdateSourceTrigger=PropertyChanged}" Grid.Row="0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="0,0,0,0"  MaxLength="20" BorderThickness="0">
                                <TextBox.Resources>
                                    <VisualBrush x:Key="HelpBrush" TileMode="None" Opacity="0.3" Stretch="None" AlignmentX="Center">
                                        <VisualBrush.Visual>
                                            <TextBlock FontStyle="Italic" Text="搜索"/>
                                        </VisualBrush.Visual>
                                    </VisualBrush>
                                </TextBox.Resources>
                                <TextBox.Style>
                                    <Style TargetType="TextBox">
                                        <Setter Property="Height" Value="25px"/>
                                        <Setter Property="VerticalContentAlignment" Value="Center"/>
                                        <Setter Property="HorizontalContentAlignment" Value="Center"/>
                                        <Setter Property="Background" Value="Transparent"/>
                                        <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>