<TextBox Name="account" GotFocus="account_GotFocus" LostFocus="account_LostFocus" Style="{StaticResource LabelTextBox}" xl:ControlAttachProperty.Label="用户名:" Foreground="Black" Margin="10,10,273,271" Width="Auto" />

<Popup Name="pop3" AllowsTransparency="True" StaysOpen="True" PopupAnimation="Fade" IsOpen="False" PlacementTarget="{Binding ElementName=account}" Placement="Right" >
  <Label Style="{StaticResource tipLable}">
    <StackPanel Orientation="Horizontal">
     <Label Content="请输入用户名"/>
    </StackPanel>
  </Label>
 </Popup>

<Style TargetType="Label" x:Key="tipLable">
  <Setter Property="Template">
   <Setter.Value>
    <ControlTemplate TargetType="{x:Type Label}">
     <Grid>
      <Border CornerRadius="4" BorderBrush="Black" BorderThickness="1" VerticalAlignment="Top" Margin="8.5,0,0,0" Background="Yellow" HorizontalAlignment="Left" Padding="5">
       <ContentPresenter />
      </Border>
      <Canvas Width="10" Height="10" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="0,10,0,0" Background="Transparent">
       <Path Stroke="Black" StrokeThickness="0.5" Fill="Yellow">
        <Path.Data>
         <PathGeometry Figures="M 10,0 L 10,0,0,5 L 0,5,10,10"/>
        </Path.Data>
       </Path>
      </Canvas>
     </Grid>
    </ControlTemplate>
   </Setter.Value>
  </Setter>
 </Style>