<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>