Using a DrawingBrush as an Opacity Mask
<Window x:Class="Workspace.DockExample"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Workspace" Width="640" Height="480">
<Rectangle Height="150" Width="200" Stroke="Black" StrokeThickness="1">
<Rectangle.Fill>
<DrawingBrush>
<DrawingBrush.Drawing>
<GeometryDrawing>
<GeometryDrawing.Brush>
<RadialGradientBrush>
<RadialGradientBrush.GradientStops>
<GradientStop Offset="0" Color="Black"/>
<GradientStop Offset="1" Color="Transparent"/>
</RadialGradientBrush.GradientStops>
</RadialGradientBrush>
</GeometryDrawing.Brush>
<GeometryDrawing.Geometry>
<RectangleGeometry Rect="0.05,0.05 0.9,0.9" />
</GeometryDrawing.Geometry>
<GeometryDrawing.Pen>
<Pen Thickness="0.1" Brush="Black" />
</GeometryDrawing.Pen>
</GeometryDrawing>
</DrawingBrush.Drawing>
</DrawingBrush>
</Rectangle.Fill>
</Rectangle>
</Window>
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