DropShadowEffect
cannot be applied to a Window
. Instead, if you want to override the default window appearance, you have to apply the effect to some other element contained in the window:
<Window x:Class="WpfApplication2.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" WindowStyle="None"
AllowsTransparency="True" Background="Transparent">
<Grid Margin="20" Background="Red">
<Grid.Effect>
<DropShadowEffect BlurRadius="15" Direction="-90"
RenderingBias="Quality" ShadowDepth="2"/>
</Grid.Effect>
...
</Grid>
</Window>