为了能实现PlaceHolder的效果，用[https://www.cnblogs.com/catzhou/p/12658478.html]([WPF 学习] 12.动态绑定VisualBrush)的方式，实在是十分麻烦，要为每个文本框都折腾一把，实在不爽。现在介绍个简单的方法，借用Tag属性传递PlaceHolder的内容，代码如下：
<Window.Resources>
<system:String x:Key="PlaceHolderEn">English</system:String>
<system:String x:Key="PlaceHolderCn">中文</system:String>
<ControlTemplate x:Key="TextBoxTemplatePlaceHolder" TargetType="TextBox">
<Border x:Name="border" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" SnapsToDevicePixels="True" CornerRadius="15" Padding="5,0">
<Grid>
<ScrollViewer x:Name="PART_ContentHost" Focusable="false" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
<TextBlock x:Name="PlaceHolder" Text="{TemplateBinding Tag}" HorizontalAlignment="Center" VerticalAlignment="Center" Opacity="0" Background="Transparent"></TextBlock>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="Text" Value="">
<Setter TargetName="PlaceHolder" Property="Opacity" Value="0.3"></Setter>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Window.Resources>
<StackPanel Name="abc">
<TextBox FontSize="40" Text="" Tag="{DynamicResource PlaceHolderEn}" Name="aaa" Template="{StaticResource TextBoxTemplatePlaceHolder}"/>
<TextBox FontSize="40" Text="" Tag="{DynamicResource PlaceHolderCn}" Template="{StaticResource TextBoxTemplatePlaceHolder}"/>
</StackPanel>
</Window>
哦，顺便弄了个圆角，那叫一个方便，爽！