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>