WPF输入框里面加文本提示

 <TextBox x:Name="txtSampleNo"  Width="265" Height="36" VerticalContentAlignment="Center"  HorizontalAlignment="Left" VerticalAlignment="Center" Margin="10,0,0,0"   >
     <TextBox.Resources>
         <VisualBrush x:Key="text1" Opacity="1" TileMode="None" AlignmentX="Left" Stretch="None">
             <VisualBrush.Visual>
                 <TextBlock Text="输入样品编号" FontSize="16" />
             </VisualBrush.Visual>
         </VisualBrush>
     </TextBox.Resources>
     <TextBox.Style>
         <Style TargetType="TextBox">
             <Style.Triggers>
                 <Trigger Property="Text" Value="{x:Null}">
                     <Setter Property="Background" Value="{StaticResource text1}"/>
                 </Trigger>
                 <Trigger Property="Text" Value="">
                     <Setter Property="Background" Value="{StaticResource text1}"/>
                 </Trigger>
             </Style.Triggers>
         </Style>
     </TextBox.Style>
 </TextBox>