Border Class

原文链接：Border Class
Namespace:System.Windows.Controls
Assembly:PresentationFramework.dll

Draws a border, background, or both around another element.

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弹出框阴影效果：
XAML代码：

<Window x:Class="PopupDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">

    <Grid Background="LightBlue">
        <Grid Width="300" Height="300">
            <Grid.RowDefinitions>
                <RowDefinition Height="50"/>
                <RowDefinition Height="50"/>
                <RowDefinition Height="50"/>
                <RowDefinition/>
            </Grid.RowDefinitions>
            <Button Content="注册" Grid.Row="2" Width="100" Height="23" HorizontalAlignment="Right" Click="Button_Click"></Button>
        </Grid>

        <Popup AllowsTransparency="True" Placement="Center" StaysOpen="False" x:Name="popup">
            <Border Background="White" CornerRadius="20" Margin="20">
                <Border.Effect>
                    <DropShadowEffect BlurRadius="15" Direction="-90" RenderingBias="Quality" ShadowDepth="2"/>
                </Border.Effect>
                <Grid Width="300" Height="300">
                    <Button Content="弹窗测试" Grid.Row="2" Width="100" Height="23"></Button> 
                </Grid>
            </Border>
        </Popup>

    </Grid>    
</Window>

效果图：

备注：

必须有这两个才能显示出该效果。