效果：

WPF中貌似不能像winfrom里一样直接加提示语，需要使用TextBox.Style，将Trigger标签插入进去。
贴源码：

<WrapPanel Name="TakeOverExpressNo1"><Label Content="物流单号："></Label><TextBox Grid.Row="9" Grid.Column="1" x:Name="txtTakeOverExpressNo1" Width="250" KeyDown="txtTakeOverExpressNo1_KeyDown"><TextBox.Resources><VisualBrush x:Key="HelpBrush" TileMode="None" Opacity="0.5" Stretch="None" AlignmentX="Left"><VisualBrush.Visual><TextBlock FontStyle="Italic" Text="输入后请按回车"/></VisualBrush.Visual></VisualBrush></TextBox.Resources><TextBox.Style><Style TargetType="TextBox"><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>
</WrapPanel>