使用样式实现带水印文本框

摘自：http://social.msdn.microsoft.com/Forums/vstudio/zh-CN/d565c79e-6b4a-44e2-a566-a4a622eab22f/how-to-set-watermark-for-textbox-in-simple-way

带水印文本框的例子：

<Window x:Class="SampleApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="500" Width="700"        
        >
    <Window.Resources>
        <Style x:Key="MyWaterMarkStyle" TargetType="{x:Type TextBox}">           
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type TextBox}">
                        <Grid>
                            <Border Background="White" BorderBrush="#FF7D8683" BorderThickness="1"/>
                            <ScrollViewer x:Name="PART_ContentHost" Margin="5,0,0,0" VerticalAlignment="Center" />
                            <Label Margin="5,0,0,0" x:Name="WaterMarkLabel" Content="{TemplateBinding Tag}" VerticalAlignment="Center"
                               Visibility="Collapsed" Foreground="Gray" FontFamily="Arial"/>
                        </Grid>
                        <ControlTemplate.Triggers>
                            <MultiTrigger>
                                <MultiTrigger.Conditions>
                                    <Condition Property="Text" Value=""/>
                                </MultiTrigger.Conditions>
                                <Setter Property="Visibility" TargetName="WaterMarkLabel" Value="Visible"/>
                            </MultiTrigger>
                            <Trigger Property="IsEnabled" Value="False">
                                <Setter Property="Foreground" Value="DimGray"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
 <Grid >
<TextBox Style="{StaticResource MyWaterMarkStyle}" Height="25" Tag="Water mark"/>
 </Grid>
   
</Window>